I thought it might be helpful to show how underglaze colors change during firing, using the previous picture of the sabino Finn.
The underglaze on this Finn is raw, which means it has not yet been fired. As I mentioned in the previous post, raw underglaze is very opaque so at this stage the horse will look very flat. Not only is raw underglaze extremely opaque, but the color is quite muted compared to what it will look like in its final form. Just how opaque the color is, and just how pale it is, varies with each color. For this particular horse, I've used a lot of a certain shade of gray that, while it fires to a rather medium shade, is among the most extreme in opacity and "frostiness". (I think of the colors that dry powdery white as being "frosty" because they look like the color has a layer of dusty frost over it.) All unfired colors are extremely misleading, but because this particular mix used a lot of that gray it will be more misleading than most.
Here is our Finn after that gray color was fired. Not only is the tone warmer and darker, but you can begin to see some of the shading on his hindquarters.
This shading will become even more noticable when the final (gloss) glaze is applied and fired. He's darker now that the paint is no longer raw, but the color will deepen even more when it interacts with the clear glaze.
I suspect that most ceramic artists, if they work with underglazes long enough, get to the point where they can automatically translate what they see in the raw, and in the fired (but unglazed) bisque, and know what they are getting as a final color. Or perhaps I should say, they come to know what their possible range might be. One of the added challenges with ceramic underglazing is that it can be said that you suggest what colors you might like, but in the end the kiln decides the exact shade!