The r/p/g/r/p is where I'm confused.
You say, "12 causes it to be R2C1 or R2C5 and that means red is in the empty spot R2C1 or R2C5"
So pick one of those corners for each color, then deal with ||| R2C2 or R2C4 <-- they don't need to be red or pink, necessarily.