creating color by their values is -- afaik -- a very normal thing in page layout.
normally, you will have a graphical chart which will be listing all the colors that your allowed to use and where you can use them.
for the times you're more free, you can use the colors from one of the colors palettes delivered with scribus or create your "own" colors with "extra > color wheel".
of course you can use the color picker, but it's really not very precise (you often get an average color).
but i have tested it: when i use the color picker, i can simply type a name and then use it right away from one of the color tabs in the properties.
finally, as i already wrote in a previous post, there is no need to use the picker for getting the color from a shape, since the only way to apply a color to a shape is to pick it from the list of colors (except if you have a gradient... but you would mention that if this would have been case, wouldn't you?)
so: what exactly are you trying to do?