How to set the colour of a texture, but only within certain UV Coordinates?

I have a texture that is applied to a mesh in my scene.
A part of the texture is greyscaled and I want to be able to change the colour of this within code.
It is simple enough to change the colour of the entire texture with renderer.material.color = new Vector3(r,g,b); However, in this situation only half of the texture if grey-scaled, and the rest has its colour already set.
I want to be able to set the colour of the part of the texture which is grey-scaled.