Bill's final sentence is what I'm aiming for. I realize the differences that various clays and firings have on the end result and I'm not looking for a perfect replication of a color. I too, have spent years on glaze testing and color theory. I'm not trying to invalidate the years of work that potters have done, just move my own in a different direction.
I've made a chart of the underglazes I have. I also have a lot of colored pencils. I match the nearest pencil I have to the underglaze, then color in the printed design to see how the colors work together. If they don't, I start over. Colored pencils don't erase well. I'm was hoping to ease the process of designing using an image manipulation program. I've learned that Microsoft's 3D Paint will identify the RBGs from my chart, but won't save a palette of more than 18 colors. Which is more than enough. I'm also, finally, taking the time to watch GIMP tutorials.