I just read a great post, Modification, Differentiation, Adaptation and the Art Classroom , over at Artful Artsy Amy and I suggest you check it out! She has put together a categorized list of ideas to help you help your students in a fair, discreet, respectful way.
My best tip in this area has to do with students who are color blind or have trouble differentiating colors. I actually got this idea from one of the 5th grade teachers at my school a couple years ago. We used it for one of his students my first year of teaching and I just pulled out the trick a couple weeks ago when a new student started. The idea is to assign a "Color Buddy". This would be a friendly, helpful student who sits near the student with color trouble and helps them find the colors they need when it is important to the assignment.
Now go read the post over at Artful Arty Amy and add your tips in the comments!