You have to charge for it, it is an extra screen and it usually uses the most ink, plus you have to flash or even print twice and flash twice.
I used to charge that way too. But, now I've found that I'd rather print a highlight white on these prints. So now I'm charging for 4 screens. Explaining this to past customers was a little difficult at times.example, a red and blue print on black, I charge 3 screens, but if its a red, white, and blue design on a black shirt, I still only charge 3 screens.
you are right, it is more work.I used to charge that way too. But, now I've found that I'd rather print a highlight white on these prints. So now I'm charging for 4 screens. Explaining this to past customers was a little difficult at times.
I think you should charge for an additional color regardless. You still have to p/f/p the white anyway resulting in more work.