One of the challenges when painting with watercolors is that the color you have mixed looks darker when it is still wet than after it has dried. This can make color matching or mixing up more of the same color difficult. But there is a solution, which isn't tricky but does require requires a little patience as you wait for paint to dry.
First, always remember you want to use a bit more pigment when mixing the color than the dried color might suggest. Then, before you apply the watercolor to your painting itself, brush a little onto the margin of your sheet of paper. Let it dry, which won't take long, then compare it to the dried paint in your painting. Adjust your mixture as needed, adding either more pigment to make it darker or more water to make it lighter.
Don't believe this is a watercolor technique for beginners or amateurs only. The photo here shows a corner of a painting by that master watercolorist JMW Turner, in the collection of Tate Britain in London. In the museum's Turner wing, there's a 'secret' upstairs section with a display of Turner's sketchbooks and watercolors, which is where I saw this painting. I found this glimpse at his working methods fascinating.
