How do I add biomes to a procedural mesh?

I have a procedural mesh that creates terrain at runtime.

And am applying a material that uses noise to to show the 2 different biomes like this:

But I cant find a way to read the colours when I go to place objects local to those biomes.

I realise I am probably doing this backwards and should assign the biomes first then add the colours but I have been totally lost on how to do this for several days.

Any help is greatly appreciated.