My 5 year old and I have started playing a little color game called, "What color is that exactly?" I'm trying to show her that colors are not always what they seem. The ocean may seem "blue", but which blue, exactly? And is it all blue? Or is it the same blue, from horizon to shoreline? One way to hone your color discerning skills is by using a color isolator. A color isolator is nothing high tech. It's basically a small hole that one can peer through to "isolate" a particular color from all others in a scene, in order to achieve better color accuracy.
There are little art tools sold to achieve this purpose, or you can peer through the small hole at the end of your palette knife. In a pinch you can concoct a small hole by peering through your curled up fingers. Last night at the restaurant my daughter and I were peering through straws. We even made a hole in a piece of my daughter's pita bread.
The point is, by isolating a tiny portion of an object you eliminate the information overload that can often happen when painting outdoors, and it is easier to see the color and value more accurately.