It's all about the time.... and, as they say, " The devil is in the details". Therefore, small amounts of additional material usually push my price up more quickly than larger amounts of a second color. Be mindful of the inordinate amount of time small details tend to add to the design & approval phases as well as the cut & application stages.
Seen so many times where most of the clients time-consuming back and forth was over the smallest of details, not the bigger, main concept. I price based on the projected time to complete extra colors/details way more than the small amounts of material costs it will add. Most clients/buyers tend not to understand or appreciate these principals and I generally don't use 'size-based' formulas for labor. It's all about how long it's gonna take and each project tends to differ greatly so, learn to judge the time involved for each additional color.