A basic rule of thumb that feels obvious but is worth mentioning regardless: the more water you mix into your watercolor cakes or tubes, the lighter your resulting pigment will be once swiped on the page. Conversely, the less water used, the more saturated and richer the color will be.
If you're trying to create a picture with a blend of colors across the spectrum, get the lighter colors down on the sheet first, and then move onto the darker ones. For example, you could start with yellow and paint red over it, but if you try to paint yellow on top of red, it won't show up.
Have two cups of water on hand: one for cleaning off your brush in between color changes, and another for rewetting your brush while still using the same color. This prevents any unwanted color mixing.
Also keep a cloth or paper towel nearby for blotting your brush between color changes or when the tip is too saturated with water.