It is not clear to find out which tool to use
It isnt clear what you’re trying to do at all. Can you provide more details please?
What I want to do it is to put a uniform color on the image like I do when I put a Cokin or Hoya filter on the front lens when I shoot a photo. Sometimes I want to travel light with just with my camera and one lens and no bag with filters, flash and tripodb and so on.
Have you tried the Color Toning or Graduated / Vignette Filter tools? The first is color-shift oriented while the second two are more about exposure. The HSV Equalizer and the curves in the L*a*b* Adjustments tool can perform more targeted color adjustments. And there’s always the Channel Mixer, but I use it so rarely I have to review each time…
I don’t use RT every day (ART more often instead) but that’s what I remember from my RT days.
What kind of filter? A coloured filter? If just a warming or cooling filter white balance might be the thing to use. Otherwise a tool similar colour balance rgb in darktable - I’m sure RT has one but I don’t know enough about it. @lphilpot 's suggestion sounds about right.
What comes to my mind is to use the layers functionality of GIMP, two layers: 1) your image, 2) single-color synthetic image, using the alpha channel for blending. You can also specify use of the foreground or background color to fill the layer.
Welcome to the forum!
Thank you for your answers it’s worth to try, I’ll let you know what worked