There are most likely shadows and highlights on picture of doggie. You might want want to play off that with a photo app if you have one. Enlarge the photo into Black and White. You will see different shades.
The other option is to make the outline and then color with thread painting. Variegated threads work well with this technique.
I would work different black fabrics into the design to help with shading. This sounds like a fun project with a great subject for your model.