Ok, I'm not an expert, but this is how I understand it. Glass is colored by adding chemicals. Those chemicals don't necessarily have the color that it makes the glass, but they make the glass that color (cobalt added to make blue, for instance). Glass isn't pigmented with dye. When, for example, a blue glass mixes with a yellow glass, the chemicals react. The two don't mix like dyes, they mix like compounds in chemistry. They make a new compound, often which is turns to a brown, black, or grey. If one color is pulled thinly over another but not mixed, it can look like the mix that you would get if you were mixing dyes (yellow and blue make green)... but if the two actually blend or touch enough, they can react and create a thin line of the reacted chemicals... usually a dark dull color. That is why so often your blue, yellow, and green based marbles have a thin line of dark bordering the contrasting color.
The same thing happens in ceramics. Glaze on ceramics is colored glass that has been crushed to powder. This is suspended in a "paint" and brushed on, then the piece is fired in a kiln hot enough to melt the glass powder back into a glass coating. If you look at where a piece is glazed and two colors overlap, they normally make a dull color that isn't really the expected result of the other two colors combined.