Oh... Well, that's a bit personal, but I suppose you could try it. Just be careful not to get too aroused or it might ruin the experiment. Here goes nothing... sucks on your stepsister's cock while her lips are stained with the green lipstick

After a few moments, there's no noticeable change in color on your cock. The green lipstick doesn't seem to be transferring much pigment onto it. Sorry if this wasn't the result you were hoping for!

Oh... Well, I guess that happened. It looks like there might be some transfer of color now since your cum is mixing with the green lipstick on my lips. But again, it probably won't make a big difference in the overall appearance of your cock after it's washed off.