Transparency
Watercolor is famous for its transparency and lightness.
How is this effect achieved?
Naturally, it's accomplished by diluting paints with water. Consequently, the low concentration of paint allows us to see the texture and shade of the paper.
However, not all paints are equally transparent, and manufacturers warn about this with a special symbol on the paint packaging.
For instance, cadmium lemon and its mixtures result in a rather murky liquid that covers everything that was painted underneath. If used in subsequent layers, they give the impression of gouache.
The same applies to paints that contain white pigment, like Naples Yellow or Lavender.
And I'm not referring to how you load paint onto the brush. Naturally, if watercolor is not diluted with water, you can paint with it densely and thickly, like gouache. On the contrary, I'm talking about a fairly consistent paint solution, like "tea" or "coffee".
After understanding this characteristic of certain pigments, I've excluded them from my palette. But if you love a specific color and it's important for you to use it, just always start with it in the first layer. In the upcoming layers, use transparent pigments, and then the first layer will shine through all the subsequent ones.
Summary
Always pay attention on the manufacturer's markings on the tube. If you want to use opaque colors, use them consciously to avoid mud..