Hot toys plastic heads are made form PVC and other companies aren't. When tinted, it tends to look a bit dull, in contrast to styrene.
To lessen the costs of materials, the paint scheme is simplified to have a base tone over which they hand paint detail, shadows etc. The base color then comes from the tinted PVC that doesn't gives out the best properties or isn't well calibrated and isn't the most color stable polymer.
Color can be characterised in various forms. One is the HSV scale. Hue (color tint, green, blue, etc). Saturation (how intense is the color). Value (how brigth).
Some of the latest heads display a low value, low brightness and they look toned down. It could be corrected adding more opacifiers/brighteners to the plastic. Maybe they are cheaping costs or their factories are running them short... dunno.
Also they are "difficult to look at", in a sense in which they look semi transparent, just slightly. That is also a lack of opacifiers, and problem of the base color of the plastic that must be calibrated better.
I'm also not sure, but I believe their hue is a little towards the yellows in most heads. Actually PVC tends to yellow overtime in exposure to UV (as most plastics do) acquiring a yellowish brown look, however not only light affects it. Also heat and it could degrade easily the final color. So again, bad color calibration. Or maybe they are over compensating the yellowing with blue tinting...
If they were to use actual paint for the base color, instead of tinting the PVC, it'd look better as it adds little variations over the color blending, light scatering, etc. and they'd avoid the color contamination from the plastic itself. This is particularly noticeable in the press shots or expositions where they have presentation models that are hand painted and look well balanced, compared to the industrial model that you can buy
