SRM = 12.7*A430 = 12.7*e*c*1 where A430 is the absorption in a 1 cm cell at 430 nm, e is the extinction coefficient and c is the molar concentration. Making lots of somewhat shaky assumptions one can reason, therefore, that a 20 °P wort from a parti gyle mash would have twice the SRM of a 10 °P wort from the same mash (because °P, while they are not a molar concentration unit reasonably closely approximate one). Thus, if you believe in the commonly used MCU scheme for calculating color and you treat both gyles the same with respect to length of boil then you might be able to come up with rough estimates of the color by linear apportionment.