To calculate color for a parti gyle:
Multiply Lovibond by pounds for each item on your grain bill and add to get a total coloring units (TCU).
I use the formula Color in SRM = 1.4922*(TCU*0.6)^0.6859
The 0.6 comes from the amount of extract in your first beer (I figure 60% for my system). By amount of extract, I mean amount of color extracted from the mash, not the volume of the mash.
I believe that the formula is based on Beer's Law (http://en.wikipedia.org/wiki/Beer%E2%80%93Lambert_law
) though I could be mistaken.
I believe that the 1.4922 and 0.6859 were determined empirically.
If you're doing 3 beers, your second beer's color is going to have 60% of the remaining 40% of color, so multiply by 0.24 (0.6*0.4 = 0.24).
Now you've used 84% of the color units (1-(0.6+0.24) = 0.84. The third beer gets 60% of the remaining color. So that's 60% of 16% = 0.096.
First beer Color in SRM = 1.4922*(TCU*0.6)^0.6859
Second beer color = 1.4922*(TCU*0.24)^0.6859
Third beer color = 1.4922*(TCU*0.096)^0.6859
In any case, this formula works for me (ymmv).