Gimp supports RGB, Grayscale and Indexed (= palette based) Images
It can convert between them, but conversion from RGB to something else
is not reversible.
Demo:
Open lena_color.jpg.
Select
<Image>
→
Image
→
Mode
→
Grayscale
Select
<Image>
→
Image
→
Mode
→
RGB
As can be seen, the image remained in shades of gray. However, now
colours can be added.
Some effects are not available in all modes
It is recommended to work in RGB or in Grayscale and convert it to
be a palette-based one only before publishing it. (and keep a copy of the RGB
source)