You can easily Repaint other artists' artworks by tapping on the pencil icon right under the image. You will notice that not all images have the pencil icon. This happens when the original artist did not enable the repaint feature.
You can enable or disable the Repaint option inside your profile. Just go to My Profile, then tap on the gear icon on the top right corner. There's a checkbox inside, which you can check to allow others to repaint your original imported artworks!
Please remember that repaint is limited to Colorfy Plus subscribers only.