filtering colors from an image

hello wonderful forum! I am playing with an image sampler component in my latest exploration and trying to have it build geometry based on color... so for instance: if the pixel sampled is within a range of reds make a triangle and if the sample is within a range of blues make a circle and so on. But according to this site http://www.rapidtables.com/web/color/RGB_Color.htm

which explains rgb values...

The red, green and blue use 8 bits each, which have integer values from 0 to 255. This makes 256*256*256=16777216 possible colors.

so does anyone have any idea how to set a range of colors and tell whether the pixel sampled is true and falls within a range determined or is false and does not fall within the range. As always a BIG thank you. -Ethan