Sometimes they are hard to explain, check step chart in the image below.
Well, given that for this kind of moiré to exist, there have to be 2 regular sampling structures with different sampling frequencies and/or at an angle , one is the camera's sensor so the other must be a non-continuous tone print technique, a raster or half-tone screen print. They state on their website
that the target has a true physical resolution of 1500 dpi, that is dots
-per-inch. Unclear is whether they mean a printing process with that fine a raster, or that they mean a measured effective resolution of 1500 samples per inch (or PPI, pixels per inch). Either way, there apparently is a raster structure that exibits interference with the camera's sensor grid (maybe Stefan Steib, or someone else who has that chart can confirm if there is a visible printed raster pattern that triggers the moiré).
The false color moiré is caused by the different sampling densities of Red/Blue versus Green, caused by the Bayer CFA, hence different sized aliases for those colors.
There is an other general comment that can be made about that specific test chart, and that it is paradoxically not optimally suited for accurate testing of discrete sampling systems such as digital cameras, or scanners (unless they specifically wanted to create a problematic test surface, not something to quantify resolution with). The sharp edges will unavoidably result in aliasing artifacts, just like reproduction of text documents would (for which purpose this target would be a good stress test). That's for example why I designed my star test target for resolution testing with a sinusoidal radial grating, and not bi-tonal sectors.
Also the 1951 USAF five (or six) bar patterns (if you count black and white as separate bars) obviously stem from the analog film days, some 62 years ago, for which it was good, but not as well suited for regular discrete sampling devices. Also Imatest's creator Norman Koren explains
that quantitative MTF tests on bi-tonal bar-patterns need to be corrected by a factor of approx. 78.5% because of the influence of aliasing from the sharp bar edges.
Here is an example from Norman's site which visually clearly shows how bi-tonal bars
mis-behave much more than sinusoidal patterns at the same spatial frequencies:
What I speculate a bit is that sometimes detail we see are actually aliases, so aliases may enhance an image, giving for instance structure to fur or feathers.
Only if they seemingly correspond with the pattern they cannot really resolve. But at an angle, aliases often produce curved patterns where straight patterns are to be expected. That can look un-natural.
I would speculate that the reason that I often (almost always) find color aliases in test images and seldom in real images is that test images tend to have more high contrast detail. Also I tend to stop down a lot for DoF.
Yes, most tests attempt to push to the limits of e.g. resolution. Because low contrast detail vanished first, one often uses higher contrasts to come closer to the real physical limit (the Nyquist frequency) beyond which no detail exists and only aliasing will be created. The false color aliasing is due to the differing sampling densities of the different colors of the Bayer CFA.