Board Thread:Questions and Answers/@comment-15974427-20140125213508/@comment-9531161-20140125214631

Well, if you're willing to use the GRP, you could use GPUTCHR to place sprites that are behind the window on the GRP, then alter the colors on the GRP (after GPUTCHR) so that they're whiter (as if you're looking through glass). However, this can be very slow, and I wouldn't recommend it.