I guess it would be color noise. I just don't understand how one pixel can register a certain color and the pixel right next to it seemingly register a different color when the background is the same. The background doesn't look smooth and uniform, it looks like there's lots of variations in it. Some of the pixels look redder than others and whatnot. You know? And even though it is low-key, its at ISO 100! I wouldn't think this would cause issues at such a slow speed (but its there regardless of ISO).
Basically the problem is that pixels aren't perfect. Some are more sensitive than others to different colors of light. And, unless you have a very specific type of detector, the pixels are generally arranged in groups of 4 forming one "macro pixel:"
R G
G B
So one out of the four records red, one records blue, and two record green. It's the camera's processor that then interpolates and figures out what the "real" color at each location is. This is also part of why if you take a photo of the moon and look at the R, G, and B channels individually, G usually looks much better than the other two (2x as much information).
So that's part of how you get color noise. The other part is the first thing I said, that pixels are not perfect. Some are more sensitive than others, etc.
You also have a degree of uncertainty governed by Poisson statistics. If 10 photons hit a pixel, the uncertainty in that count is SQRT(10) ≈ 3. So two pixels right next to each other that receive the exact same amount of light may record very different intensities - one may record 7 and one may record 13, almost twice as bright. If you record more light like 10000 photons, then the uncertainty is SQRT(10000) = 100, so those two pixels may record 9900 and 10100, which is only a 2% difference.
It doesn't matter what the ISO is for this - you're still going to get these processes occurring ...
Unless you had the EXACT SAME amount of light falling on each of the individual "pixels" of the image, then yes its not going to be "uniform".
... hence why this statement is not correct.