I have done quite a lot of work in collage. I find it helps me to be bolder with my colors and to simplify shapes while still allowing visual interest with the surface textures. I can try out color juxtapositions before gluing them down and sometimes discover combinations of colors that I might not have thought of but that really sing.
This close up shows how the background is made up of pieces that have been carefully cut to fit to each other while the giraffes and the border shapes were added on top. I usually find bits of old paintings to cut up though sometimes I paint the paper specially. Because I'm using watercolor paper the end result is quite thick and I will often use a piece of mat board to glue the whole thing down on so that it doesn't buckle.