advocate COLORTERM=24bit for linux console

TERMS.md

@@ -14,7 +14,7 @@ The following have been established on a Debian Unstable workstation.
 
 | Terminal | Recommended environment | Notes |
 | -------- | ------ | ----- |
-| Linux console | `TERM=linux` | 8 (512 glyph fonts) or 16 (256 glyph fonts) colors max, but RGB values are downsampled to a 256-index palette (see below). |
+| Linux console | `TERM=linux` `COLORTERM=24bit` | 8 (512 glyph fonts) or 16 (256 glyph fonts) colors max, but RGB values are downsampled to a 256-index palette (see below). |
 | FBterm | `TERM=fbterm` | 256 colors, no RGB color. |
 | kmscon | `TERM=xterm-256color` | No RGB color AFAICT, nor any distinct terminfo entry. |
 | XTerm | `TERM=xterm-256color` `COLORTERM=24bit` | Must configure with `--enable-direct-color`. `TERM=xterm-direct` seems to have the undesirable effect of mapping low RGB values to a palette; I don't yet understand this well. The problem is not seen with the specified configuration. Sixel support when built with `--enable-sixel-graphics` and run in vt340 mode. |