John Singer Sargent said he carried watercolors "in case of emergencies". I have a love/hate relationship with watercolors. On one hand they are immediate, portable, and make beautiful marks on the paper. On the other hand I find them unforgiving and nearly uncontrollable! I often carry watercolors for "emergencies" and they can be fun until they become frustrating. I've long since given up the notion I'll ever master this medium, but I do enjoy sketching outdoors with them occasionally.
