2014 D Lincoln Penny Coloring

Coins can attract many different colours from the environments they get put in, this colouring effect is called "toning"

Toning can add a premium for attractive colour, especially for coins that already have value like Uncirculated Morgan dollars, but in this case it wont affect the coins overrall value, which is unfortunately one cent.

The other factor is the final rinse the coins are put thru. If that solution has gotten dirty, then it will dry leaving that crap residue to effect the coin's surface. Yellow in th scale of metals, shows as Brass, which is a lightening of the Bronze alloy applied as Copper plating. Bronze being a product of copper mixed with zinc and a few slight trace elements. The more zinc, or tin even, will turn to the yellow scale. If materials in the solution have a high zinc content, it is possible that this can change the surface content ever so slightly. It will be a permanent effect, not removed after the damage has been done, just like environmental staining of the plating.