Watercolor is a challenging medium to work with to be successful you really need to learn your palette and several key factors of how the paints work together.
While learning to use watercolors, do you find yourself creating a lot of muddy color? If so, one of the factors could be the use and mixing of opaque watercolors. I am not against using opaque colors, but it is much easier to create mud if you do use opaque colors. The reason is mixing two or more opaque colors together or mixing one opaque color with too many transparent colors. The colors are brilliant on their own but too many opaques can ruin a painting.