Shotgunworld.com

Why are most of their stocks
So dark redish?

And

How do they do that?

Coles stocks are oil finished, not polyurethane, so the finish of the stock comes out darker, and depending on the grain, red'ish as well. If needed, a bit of staining is used to unify the butt stock to the forearm.

It all comes down to the wood that is being used, and in there case, it's upper grade wood that is being presented.

Getting linseed oil to give a red hue is done with alkanet root. Grind some alkanet root in artist grade linseed oil and it will go red, like brandy. It gives the wood a red hue and also makes the figure darker, giving more contrast.

CCL stock oil, made by British gunmaker Clive Lemon has alkanet.