YouTubers 2GuysTek was able to fit a DIY RGB fan frame into the Noctua NF-A12x25 and the results were phenomenal, at least aesthetically. There could be performance impact in terms of acoustics as they had to saw off the original inner fan frame and replace it with their 3D printed custom frame. You can watch their YouTube video below to see how they did it.
On the other hand, the frame of the fan is using what Noctua calls Advanced Acoustic Optimisation (AAO), in which the chosen plastic for the build of the frame has the ability to absorb sound and vibration effectively.
In the end, this project just shows that having RGB Noctua fans are possible but have to DIY it yourself since Noctua stated that they are not planning to release RGB fans any time in the future.
If the brown fans aren’t for you, the Noctua Chromax line is a good alternative, or even external RGB fan frames that won’t require you to gut down the fan itself will work just fine.