I want to add text to a photo with multiple colors (from shining light to black) so can't use one color. I want to put a poem on a background picture, so a bit long, so no way to avoid all of the colors.

I used gimp to make sigs for 1.5 years but that was about a year ago and I'm pretty sure I remember some way to invert the color to be the opposite color of the picture it was on.

Either what i remember a little, or something else to fix my problem would be greatly appreciated.

tl;dr- I have a large photo and i want to add quite a bit of text to it.