Whenever I visit with friends about my gardening book, I always encourage purchasers to use some colored pencils or watercolors and make the book their own.
Some are reluctant for fear of "messing it up" or making a mistake. Here's an idea: make a photocopy of the page you'd like to color and try different approaches.
In the example below, I used colored pencils for the paths, stones, and soil, and I used watercolors for the plants. Be careful with markers, since they might bleed through to the other side.
Just have fun.