Since September 11, 2001, the U.S. Army and U.S. Marine Corps have changed camouflage patterns for their combat uniforms twice. First it was the adoption of digital patterns, then the current move to MultiCam. It was SOCOM (special operations command) troops who first had second thoughts about digital. The digital camouflage pattern uses "pixels" (little square or round spots of color, like you will find on your computer monitor if you look very closely), instead of just splotches of different colors. Naturally, this was called "digital camouflage." This pattern proved considerably more effective at hiding troops than older methods. For example, in tests, it was found that soldiers wearing digital pattern uniforms were 50 percent more likely to escape detection by other troops, than if they were wearing standard green uniforms. What made the digital pattern work was the way the human brain processed information. The small "pixels" of color on the cloth makes the human brain see vegetation and terrain, not people. One could provide a more technical explanation, but the "brain processing" one pretty much says it all.
Another advantage of the digital patterns is that they can also fool troops using night vision scopes. American troops are increasingly running up against opponents who have night optics, so wearing a camouflage pattern that looks like vegetation to someone with a night scope, is useful.
But digital doesn't rule, at least not when price is no object. The runner-up in the competition was a non-digital pattern called MultiCam (cleverly designed to hide troops in many different environments). Many in the army preferred this one, but the difference, in tests, between it and the winner, digital ACU, was not that great. Moreover, MultiCam was about three times more expensive.
However, SOCOM operators have their own budget, and had many of their guys out in the field wearing MultiCam, rather than the digital ACU. Now SOCOM has always had a larger budget, per capita, than the rest of the army, and its operators had a lot of discretion to use whatever weapons or gear they thought best for the job. Apparently, on some jobs, MultiCam was considered more suitable than digital ACU. That said, there have been few complaints from soldiers about ACU, which measures up to MultiCam in most particulars, and it a lot cheaper.
Eventually, the services decided that if MultiCam provided even a small advantage over digital, than MultiCam was the way to go. The British Army thought the same thing, and are now sending new uniforms, using a version of MultiCam, to their troops.