Mylar sails are easy to apply graphics to. Pretty much any kind of vinyl adheres and stays well and any graphics shop can full color print and/or contour cut solid color vinyl graphics or you. (including mine). Dacron sails are a bit questionable in this regard. I've had vinyl hold up well and I've had it not do so well. The best bet for attempting vinyl on a dacron sail is to apply it when the sail is new and crispy. The sticky backed dacron (like is used for sail numbers) is the best choice on dacron sails. I can get it in 54" rolls but it's about four times the cost of vinyl.

For spinnakers, if you can't do the graphic with an old projector and a couple of different colored sharpies, order it with the graphic. Nylon (especially slippery coated nylons) are almost impossible to get anything to stick to.