Which Would Add More Value, Colour Or Scarcity?

2 million and 1 million only mild scarcity for both, and if NCLT, not even scarce.
If both new issue, don't let the marketing people get at you.
In terms of value, the numismatic aftermarket market will determine the relative value for you.
In this situation, just buy what you like the most.

Since it's a new circulation item and in the quantities you quoted, I wouldn't consider it an investment item.
Go with the one that would please you more.

Obviously non-colorized, unless the coloring is really pretty, in which case I'll try to get both.

Now if the mintages were inverted, with the colorized version less common (as is in fact usually the case), I'd have seriously considered it...

Over the last five years or so, the Royal Australian Mint has been issuing Two Dollar colorized coin into circulation with mintages in the two million range.

Just enough for the non numismatic general public to perceive them as as rare, (although at two million mintage, they are not), and pull them out of circulation for safe keeping.
A tidy little profit for the Mint, because they cost a lot less than two dollars each to produce.

I admit: I am somewhat of a sucker for collection of two examples of each at face value, but beyond that, I just recirculate any that I find.
I certainly don't see them as an investment, but a loss for me at two dollars per coin. That's the price of collecting them.

Such coins can sell for double face value or more amongst collectors, but the Mint has already taken it's nice little profit.

I collect coins primarily, those meant for circulation and their proof variant meant for collectors. So personally, if it were a particular commemorative I may buy it colorized regardless of mintage total. If I were buying as an investment or profit I would almost certainly buy the colorized version. World coins have been offered in various colored versions for years, their offerings increasing drastically in the last three to four. The market has exploded to the point where coinage has changed to meet this new market. For instance, Canada offers a glow in the dark coin, Australia has gem embedded coinage and England has photographic coinage. And the United States finally realized this with the release of it's first non-tradition coin, the 2020 Basketball.