A key in learning to paint color vibrantly is observation. When we are young we learn that the sun is yellow, tree trunks are brown, the leaves are green and the sky is blue. After we learn this elementary approach to color we tend to stop looking. We really don't observe in large part how light affects everything. The local color of that tree trunk may be brown (we will term colors differently in the future, but for now I'll say "brown"), but take a closer look. Look at the side the light hits. Now I'm going to say one of those "Jimisms." Don't say what you think you see, but what you really see." In other words, really observe what you see and throw out your default that says that tree trunk is brown. Look. Chances are where that sunlight hits the trunk there is sections that are yellowish, orangish...maybe even some pink. I know, you are thinking I'm crazy by now, but really look. Stare at it. And check out those shadows. Not just dark brown. You may see cool shades of blue, purple, green - yes maybe muted blue, purple or green but nonetheless colors that would be best described by those terms!
It takes awhile and concerted effort in observing to start to see the vibrancy of colors all around. And yes, at first I thought my teacher was a little loony at first until it clicked one day and I too, started to really observe color in objects. Many an artist has described this as "scales falling off the eyes." All of a sudden what was not seen is in clear view.
Observing - it is a key to much as we learn to paint pictures of the world around us with realism and vibrancy.
And that reference to 'scales falling off the eyes," - that is a direct quote from the Apostle Paul's life - when he was blinded by the light as he encountered the risen Christ, and then was healed and given his eye sight back. Powerful stuff.