I've had a lot of questions about this topic. I'm going to put on my teacher’s hat and talk about watercolor paints and their particular characteristics. The reason I am spending time on this topic is that I want to share information about "negative" painting, and it is necessary to understand opaque and transparent for this technique to work.
Transparent: permits light to penetrate. Allowing the white surface of the paper or underpainting to show through. These pigments are fine in texture. When multiple glazes of transparent colors are painted on top of each other (when dry) the color beneath will show through changing its appearance.
Opaque: impenetrable to light; not allowing light to pass through. These paints are relatively chalky in appearance when dry. All cadmiums are opaque. Semi-Opaque or Semi-Transparent: These paints are “middle of the road”. These paints are generally safe for glazing techniques.
Opaque or Transparent how do you know?This information is available from manufactures and can be accessed in catalogs, art supply stores and the internet. I have spent a sizable amount of time collecting manufacture information. Not everyone has time or interest to do this research. The quickest and best way to determine if your paints are opaque or transparent is to do a simple test. With a permanent marker (Sharpie Chisel tip) draw a bold line across a piece of 2x2 inch watercolor paper. With paint the consistency of whole milk paint over the line. If the black line is obscured (chalky looking) it is opaque. On the paint swatch I write information (brand, color name, index code). I keep all my color swatches in a binder for future reference. Over the years I have built an extensive binder of color swatches. I am amazed how often I refer to this information.
I don’t want to give the impression I am against using opaque paints… I like them but I don’t keep them on my palette as a main color. When I need an opaque color I squirt out some fresh paint. I do a lot of “negative” painting built up with glazes, and glazes get “muddy” with opaque paint. Opaque are great for accent marks at the end, but not for glazing. Glazing is when you paint a transparent layer on top of a dry layer. Opaque paints will look chalky or muddy if used for glazing. If you have a tendency to get “muddy” colors chances are you have an opaque paint on your palette.Look closely at the paint swatches below. Look at the top row and notice how the paint seems to be sitting on top of the line=this means opaque.
Look at the bottom row, notice how the paint does not effect the black line=this means transparent.
I encourage you to try this with your current palette.