change colour from tan to black

[TheBadger]

I want to change a colour to black (including all shades). If a part of my picture is red or tan, for example, I want to change it to black (like for a car or T-shirt). 

I've seen tutorials for colour changes using Rotate Colors, but these just handle RGB colours. 

All answers online do not address my issue, or they just assume I'm looking to change my pic to black and white. 

Thank you

[denzjos]

You can try this :
Use script sg-decomp-to-layers.scm: Filters / Colors / Indexed Decompse to x Layers (try 24 or more)
A new image is made with (in this case) 24 layers of color
Set in this new image : Image / Mode / RGB (set mode to RGB)
Copy the original image to the new image set it as layer 1 in the new image ( below the 24 generated layers)
Make all layers invisible except those you want to color black.
Use : Colors / Desaturate / Mono Mixer (play with the sliders to make the chosen layer(s) color black)
Merge the layers