This is not my original idea, but I wanted to post an example of this really cool looking technique that I tried this weekend.
Step 1: Stamp two images with waterproof ink or emboss with embossing powder.
Step 2: Color one image in with markers, colored pencils or watercolor pencils. If using watercolor pencils, blend with water or a blending pen.
Step 3: Cut out any shape from any area of the colored card you wish and mat with a piece of cardstock cut to the same shape.
Step 4: Align smaller images onto your original image using your favorite adhesive or with foam tape to give it even more dimension.
Two more examples for you, the heart has foam tape underneath it to add some height to it, and the angel card is attached without the foam:
You can also do the reverse of this technique by coloring in the bottom image and keeping the top image plain like this example here. I used Twinkling H2Os to color the image of the light post to give it the added sparkle.