Color Range configurations are stored in the DSDACR lump. These defaults can be changed by replacing the lump. The ranges are used to translate font characters into different colors for the extended hud and menu text - manually created translation tables are no longer needed. Since the final colors are still constrained by what is available in the palette, custom color ranges may be preferable to the defaults (for instance, if a palette has no reds but has a rich range of pink).
There is an exception for palette index 176, which is always translated to the upper limit of the color ranges. This is a current limitation of the extended hud.
The first line of the DSDACR lump specifies reference lumps to use for the dynamic range calculation:
doom_reference heretic_reference hexen_reference
By default, these are set to the "A" character of the message font. You may want to change this if you have a lot of variance in the color range of different font characters. The reference lump is scanned to get an idea of the level of contrast in the font.
Each line of the DSDACR lump is plain text in the following format:
color_range_index r1 g1 b1 r2 g2 b2 comment
The color_range_index specifies which range will be replaced. The color range extends from (r1, g1, b1) to (r2, g2, b2) in the rgb color space. The comment field is optional and only exists for convenience (e.g., to label the colors). You can find the current default configuration here.