I actually have this same card and I’d love to see the answer. My rgb ram and my 7800 are both in rainbow vomit mode, my Corsair front intake fans are the only thing I was really able to control. My machine sits under my desk, and the lighting doesn’t really bother me much, but I’d love to set everything to one static color.

Edit: I’m using Fedora 39 if it matters.

It's an issue with all AMD cards. The Linux amdgpu kernel driver does not expose the i2c interface that is used to control the RGB, so even if we have support for the card in OpenRGB on Windows, it won't work on Linux at the moment. The problem is their driver heavily links i2c ports with display connectors and the RGB is always on one of the otherwise unused i2c ports. I think AMD GPUs have 8 i2c ports on the chip and most graphics cards have only 4 or 5 display connectors that each use one i2c port. The Linux driver only exposes those 4 or 5.

I have the Sapphire 7900XTX nitro+ and i did the following, might be the same for the 7900XT model.

It has an external A-RGB control header that you can plug into an A-RGB port on your motherboard or controller. (cable is provided)

The only caveat for linux users is that you need to use the Trixx app on Windows to change the card lighting to be controlled using an external source. This will set the control mode and persist trough reboots.