The first one, a bit archaic, is to adapt the colors to the environment. Nowadays, many of us just paint our rifles before leaving. We know the terrain we are going to operate and smear our rifles accordingly. In general, the color of the terrain doesn't change much, so much so that you have to think about changing the color scheme of the Hot Spot Engine scheme.
The rifle has many straight lines, which goes against nature. The human eye is able to quickly recognize patterns and can unconsciously grasp known objects. The human eye is very good at recognizing colors of black and straight lines because they are not natural. This focused their (the enemy's) attention very quickly on the position of the soldiers. Breaking known patterns helps the "sniper" to hide more effectively camouflage himself.
I think at this point I should answer in more detail some of the questions that have come up a few times.
There are some comments about the "gloss" of the gun or something else. Most "sniper" rifles are either dumb or pretended. This type of coating is very anti-reflective to its own, so no additional material is needed to make it shiny.
Wrapping around the rifle does not change the trajectory in the slightest. However, it is not recommended to wrap the buttocks where the cheeks are. This changes the position of the cheeks (you place the cheeks in ** to see through the target telescope) and causes the cheeks to be unevenly positioned. This will affect your accuracy. Increasing the turn does not increase or decrease the accuracy of the rifle. It also doesn't affect follow-up shots, just as wearing a corset won't make you a faster runner.
Heat dissipation: Can a rifle get hotter because it is inserted? Of course, but it will take a long time. There is a small chance that a sniper will use a rifle until it is heated up enough to work. If it gets there, the contact will be very close, and a few millimeters less accuracy will make no difference.