First, WATERCOLORS AREN'T TRANSPARENT..!! Really...The very same pigments are used in watercolors, oils, pastels, acrylics, even house paint and lipstick. They aren't little bits of colored glass that light can pass through. Check the pigment numbers on your tubes - same PB 29 ultramarine blue in oil or watercolor.
So why do they look transparent? Because we're viewing what might be called micro-pointillism. When the paint is applied, the water is initially on the surface of the paper before soaking in. The tiny pigment particles are kept suspended by something called Brownian Motion before settling to the paper's surface. When they do, they distribute themselves fairly evenly, owing to the aforementioned Brownian action.
Like these scattered bearings, light can bounce off the white paper AND the pigment particles so it appears transparent.
Why do oils look opaque then? Because in watercolor we're looking at a THIN layer with pigment particles side by side. In oil we're looking at a THICK viscous layer with particles stacked over one another like this...forgive me... jello "salad". Try not to hurl.
Let's talk about "MUD".
In watercolor, what gets called mud is just TOO MANY SUBSEQUENT APPLICATIONS OF PAINT TO THE SAME SPOT. All those white spaces between the particles just get covered up - so it looks opaque...because it is. Look at this sample below: I went around my palette and mixed ALL 14 COLORS TOGETHER. That's the top color. It's an interesting grey that could even be used...for rocks, maybe. Point is: All 14 pigments are distributed NEXT to each other.
Below it I applied one color, let it dry and then applied the next as a glaze and so on. I never made it to all 14 colors. Sample 2 is only 9 colors but look how murky, because they're ON TOP of each other. Tap or pinch it up larger to see.
So the trick is learning to mix your colors pretty close the first time. Glazing is to get a specific effect...one color over another. It's ability to correct has limits.
Next post we'll address proper use of opaque paint in watercolor.