Small text colormap tweaks

Tackles the main issues outlined in #768 (closed), like peridot having an incorrect shade and red/blue having too much contrast. However, the proposed changes to the red and blue colormaps seemed to make the text too dark in comparison to the other colors, so this merge request tweaks them in a different way. Besides that, the green colormap has slightly reduced contrast, and the yellow colormap now uses one more shade of yellow before going into the gold range.

Here's an overview of the changes using SonicX8000's test graphic, where top is old and bottom is new: