The first is a monochromatic collage using variety of reds plus white and a little black. I also mixed a dark, almost black color by adding a dark green to one of my darker reds. I added several collaged images, but felt like it was too cluttery, so I painted it over with the colors from my monochromatic palette and added some curved fabric shapes.
For the second collage with analogous colors ranging from red to yellow, I used the same approach, laying down the colors and adding the images. I moved the images around until I got something that I liked, then pasted the images down with matte medium. I did a bit of tweaking with paint and additional images.