I have four colors :red , black ,white and brown and I want to detect them .What should I do ?

 

It would be easy using an MCU. Why the restriction?

You have listed four colors. Are these exclusive of all other colors? That is, will there be other colors such as yellow or blue?

If there are only these four colors, and presuming that they are reflective objects and not emissive, and the lighting on the objects is always the same, you can probably get away with just two sensors.

One sensor with a red filter to detect red.
One sensor that detects white and use amplitude discrimination to determine black, brown (or grey) and white.