I will be starting classes again next week and I am just reviewing my Transparent colours. It is a safe bet that the student can't make mud if you stick to transparent w/c. I find living on the west coast you are faced with alot of trees, bark and sand. Muddy looking trees definitely kill an otherwise nice painting.
If you aren't using the Quinacradone colours please give them a try. Q Gold is a must in my palette now. I love Daniel Smith Quinacradones but Windsor Newton are now stocking them too.
I have been painting along time but it is always good to refresh yourselve now and again.