Colour Mask map that where RGB channel accepts only red , green and blue colours

I can't seem to find or implement a working shader that does what i want. Basically I want to be able to have a map channel called Mask map. I want to make the mask map only accept red , green and blue colours of the map that it receives. From what i found people refer to the red,green and blue channel of the texture but i don't want to refer to those channels but rather the rgb channel will only accept red , green and blue colours it that makes sense.

For example red parts of the mask map would mean specular highlights can appear there but not on blue parts of the mask map

Is there any examples available that i could look at or if anyone could help me it would be nice