Getting image colors closer together?

[Acoustic]

If I wanted to take a picture like this (or any other picture for that matter) and make the colors more similar and the brightness of the colors more similar, how would I do that? I've exhausted my methods and have came up short.

https://ibb.co/xHrYW2r

[rich2005]

All sorts of ways and no one way will be universal for all images.

You could try: Colors -> Colorize and play with the sliders

If you find a suitable palette, one with limited colors, then Colors -> Map -> Palette Map might work https://i.imgur.com/pDXWWIh.jpg

Color pick a color, fill a layer over the image, try out various layer modes. This one LCH Chroma https://i.imgur.com/ZHBTT9J.jpg

The Gimp gmic plugin from http://www.gmic.eu has any number of ways for recolouring an image. Also has various color curves which might level-out brightness. https://i.imgur.com/PWTcBvK.jpg

All depends on the image, a portrait very different from your rendered plasma.

[Ofnuts]

Your image looks like plasma with non-red-orange colors rotated into the red-orange quadrant: