When I meet with users, I find that the vast majority have very little formal training in graphic design. Don’t read that wrong. I’m not saying they aren’t good designers, just that they haven’t gone to school to study graphic design.
Just going to school will not make someone a good designer. There are many great designers who do not have formal training. What the training can do is help you to avoid mistakes. More mistakes means you aren’t as productive as possible. It may also mean you aren’t using your creativity to its fullest.
One area where I find users lacking is fully understanding color. What do colors mean? What colors work well together? The Pantone Guide to Communicating with Color is a fantastic resource that will answer those two questions and much more.
Learning how to use design software is important. But also understanding design itself is just as important. This book is a great starting place.