2021 Quarters What Is The Dull Color?

I don't think the coins are being welded on at the mint unless you are referring to the equipment repairs,they are rather struck on a blank planchet by a Hammer under extreme pressure! In any event, the following is a good general info.
Coin Production:
Step One: Blanking. Blanks are flat metal discs that will eventually become coins. ...
Step Two: Annealing. Blanks are annealed to prepare them for striking. ...
Step Three: Washing & Drying. ...
Step Four: Upsetting. ...
Step Five: Striking. ...
Step Six: Bagging & Packaging

Article by James Buck ( the spruce craft)

The methods used to make coins has evolved over the years. Coins were first made in the ancient kingdom of Lydia well over two thousand years ago. The minting process for ancient coins was quite simple. A small lump of gold, silver, or copper was placed on a coin die embedded into a solid surface like a rock. The worker would then take a second coin die place it on top of it and strike it with a large hammer.
Medieval mints used preformed round discs of metal and a screw press to manufacture the coins. Although this was a manual process, it was easier and yielded a more consistent quality than the ancient minting process.
Modern coins are minted with hydraulic coining presses that automatically feed the blanks into the machine. When the machine is running at full capacity, rates of over 600 coins per minute can be obtained. This speed is necessary for an operation like the United States Mint that must produce billions of coins every year.
Although the process is complicated because of the automation used to produce billions of coins, there are a few common steps that every mint around the world uses. The United States Mint is the largest mint in the world, and we will focus on its production process.

1. Mining Raw Materials
The minting process begins with the mining of raw materials. Mines from across the United States and around the world supply the gold, silver, copper, or other required metals. The raw metal obtained from these mines contains impurities that are not acceptable for coinage.
In addition to the mining of ore to obtain the required metal, the United States Mint also uses recycled metal reclaimed from various sources. These sources include coins that are no longer "machinable" and are removed from circulation. They are returned to the mint where they are recycled into new coins.
2. Refining, Melting, and Casting
The raw metal is refined to remove almost all impurities. Some coins require an alloy of two or more different types of metals. The refined metal is melted, and the additional metals as required by the specifications are added. For example, the United States Mint makes its five-cent coin from an alloy of 75 percent copper and 25 percent nickel.
Once the proper purity or alloy is achieved, the metal is cast into an ingot. These are large metal bars which contain the proper amount of metal as required by the mint. The metal is checked throughout the process to make sure the right purity is obtained.
3. Rolling
The process of rolling the ingot to the proper thickness can be long and laborious. The ingot is rolled between two hardened steel rollers that are continuously moving closer and closer together. This process will continue until the ingot is rolled into a metal strip that is the proper thickness for the coin being made. Additionally, the rolling process softens the metal and changes the molecular structure which allows it to be struck easier and produces higher quality coins.
4. Blanking
The United States Mint uses rolls of metal that are approximately 13 inches wide and weigh several thousand pounds. The roll of metal is unwound and flattened to remove the curvature from the manufacturing process. It is then passed through a machine that punches out discs of metal that are now the proper thickness and diameter for the coin being made.
5. Riddling
Up to this point, the production process used to fabricate the metal blanks is dirty and is ran in a harsh environment. It is possible for small pieces of waste metal to get mixed in with the coin blanks. The riddling machine separates the properly sized blanks from any foreign matter mixed in with the coin blanks.
6. Annealing and Cleaning
The mint then passes the coin blanks through in the annealing oven to soften the metal in preparation for striking. The blanks are then put through a chemical bath to remove any oil and dirt that may be on the surface of the coin. Any foreign material can become embedded in the coin during the striking process, and it would have to be scrapped.
7. Upsetting
To protect the design that's going to be impressed on the metal coin blank, each coin blank is passed through a machine that has a set of rollers that get a little bit smaller and imparts a raised metal rim on both sides of the coin blank. This process also helps ensure that the coin blank is the proper diameter so it will strike up properly in the coining press. After this process, the coin blank is now called a planchet.
8. Stamping or Striking
Now that the planchets have been properly prepared, softened, and cleaned, they are now ready for striking. Business struck coins are automatically fed into the coining press at a rate that can reach several hundred coins per minute. Proof coins made for collectors are fed by hand into the coining press and receive at least two strikes per coin.
9. Distribution
Coins that pass inspection are now ready for distribution. Business struck coins are packed into bulk storage bags and shipped to The Federal Reserve Bank for distribution to local banks. Collector coins are placed in special holders and boxes and shipped to coin collectors around the world.

This is also educational in plane language:

https://www.google.com/search?q=how...CMtAad66sw29

What would be the reason that anybody at the mint would be welding on any coin? And I would stake my 45 years of experience in working with metal that no one could weld on any coin without destroying it.

@Cujohn You are right. And I question where in the Mint we have welding, nowhere. All the line has 4 line assistance, including the expert of the randomly checking of the batches quality.

Those are: 1. forklift for put the rolls in place. 2. annealing. cleaning computer operator. 3. quality inspection and 4. forklift operator for bring the scrap bins and strike coins bins in the wright place. Even on the mechanical workshop they do not use welding, they ere just to replace parts if and when it is necessary. Those parts are seal bags from manufacture of the line and could not be accessible with out the team present.

Old way of striking it is far behind us.

You missed my whole point. They could have passed through many many actual hands between the time they came from the mint to the time they end up in yours! You have no way of telling one way or another. You are only theorizing that no human hands has touched your $500 box! I don't know, nor do you know unless you actually witnessed the transfer of each coin to you personally.

Appears to me, there is no definitive answer to my question but great theories and great conversations, thank you all for your responses.

From what little I know and the short time I have dealt with coins, a little over a decade, I think this is relatively simple. There is nothing special about this, it is usually one of two things, when it comes to discoloration, environmental issue or an off mix in the elements. My personal feeling is it's environmental. It does not even matter if if were straight from the mint, the stops these coins make are so different in environment, it's not funny and as far as I know the trucks these are shipped in are not temperature controlled and some of the locations they go through, could be much less than controlled. These coins could have been through major transformations, hot to cold, dry to high moisture, before they ever found you and your destination, just my take.

I remember the Welding guy. He had a different profile name but same welding theory on many coins.