Transform colors. Help please

Does anybody here know how to calculate different parameters :
ra, rb, ga,gb, ba, bb, aa,ab when transforming from one color to another ?
Example: I want to change from #FFFF00 to #993300
according to photoshop the RGB for #993300 is R=153, G=51 and B=0
What is the right method to calculate these parameters?

Thanks :slight_smile: