Unlike sixels, which have a confusing layer of indirection, to change the text color, ReGIS can edit the colormap directly. Just change the colormap index you want (usually 7) and all text on the screen will be shown in the new color.
Important colormap entries for text and their defaults at power on.
| Index | H | L | S | R | G | B | Text attribute | |
|---|---|---|---|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | Screen Background | |
| 7 | 0 | 46 | 0 | 46 | 46 | 46 | Foreground Text | |
| 8 | 0 | 26 | 0 | 26 | 26 | 26 | Bold+Blink FG | |
| 15 | 0 | 79 | 0 | 79 | 79 | 79 | Bold Foreground |
If you don't want to make text unreadable, The two most important colors to not change are colors number 0 and 7 (bg and fg). Instead, use colors 1 to 6 and 9 to 14 for graphics. If twelve colors is insufficient, color 8 (bold+blink fg) is reasonably safe to commandeer. Color 15 (bold fg) can also be used in a pinch.
To change the text foreground and background:
echo "${DCS}p;S(M 7(H60 L80 S60) 0(H150 L50 S60))';${ST}"
ReGIS on the VT340 specifies colors in the Hue-Lightness-Saturation color space, not RGB.
Sidenote
Technically, there is a way to specify two-bit RGB colors, but it is not worth even mentioning.
From colormap.md, q.v.:
| Attributes | Foreground | Background | Escape Sequence |
|---|---|---|---|
| Normal | 7 | 0 | ␛[0m |
| Bold | 15 | 0 | ␛[1m |
| Reverse | 0 | 7 | ␛[7m |
| Reverse Bold | 0 | 15 | ␛[1;7m |
| Blink (off) | 7 | 0 | ␛[5m |
| " (on) | 0 | 7 | " |
| Bold Blink (off) | 15 | 0 | ␛[1;5m |
| " (on) | 8 | 7 | " |
| Reverse Bold Blink (off) | 0 | 15 | ␛[1;5;7m |
| " (on) | 7 | 8 | " |
The VT340 has no single command to programmatically reset the colormap. While it is possible to do so using DECRSTS, using ReGIS is clear and simple. See resetpalette.regis for a file you can send to your VT340.
As you can see, all text shares the same colormap entry so it is always all the same color on the screen.
It is possible to use ReGIS to fake multicolor text on the screen, but it is not what the VT340 was designed for. Please see hackerb9's discussion on fake text color.