Going back quite a bit now, but regarding the camoed QBZ 95. As far as I am aware, British troops are allowed to spray paint their rifles with an appropriate design for the environment they are operating in, I wouldn't be surprised if the pictured PLA troops are allowed to do that as well. Its a simple, cost effective and accessible solution rather than manufacturing new housings in a different colour. Perhaps in the long term, the layers of paint can get messy...
There are in fact a number of nations that do that. Generally rules are set though.
First only in deployed settings,
Second, upon return to armory the paint has to be removed hense solving the issue of layers of paint.
Third there are normally regulations on what types of paint are allowed, especially in regards to more modern weapons due to polymer materials. There are also issues of chemical interaction between cleaning agents and coatings.
Fourth the painting of the weapon requires some skill. Remember that weapons like the QBZ95 are machines. And machines are built to a set tollarence between parts. If the paint is to thick it can interfere with the function of parts like say the safety selector. Or prevent use of accessory rails (less a issue for the PLA). Some parts of the weapon as such are best left unpainted like the barrel that can heat up potentially burning the paint, moving parts like bolts, controls. Parts used for disassembly like pins and latches.
This is part of the reason why more and more military small arms have started to move away from black and to either a green or tan color. If the rifle is already in a color suited to the environment there is less a need to paint it.