Separate green, yellow and black colour grains from an image using matlab algorithm - develop

I want to separate immature(green colour), mature (yellow colour) and discoloured (black colour) grains from an image.*

Help me to create a matlab algorithm (code) so as to identify which category in numbers percentage or weight percentage.

I suggest to predict the category using the colour value of the category (lab values) followed by if, else, continue statements.

Please help me and forward ur help in vithuprabha@gmail.com