Camouflage is not universally good for military action. In the days of the musket, it would have been a very bad idea to wear camouflage: you could not see where your fellow soldiers were and how they moved, and if you moved out of synch with them you were dead. Bright colors were communication. And at the ranges of the weaponry of the day, camouflage would have made no difference anyway. You couldn't hope to "sneak on the enemy" when the smallest piece of weaponry in existence was fifty muskets. Not a musket, which was useless by itself, but fifty muskets.
Is camouflage a passing fad? Could well be - as far as "main combat" is considered. Knights had no use for camouflage. Formations of swordsmen had no use for camouflage. Yet individual horsemen or individual swordsmen could and would camouflage themselves, hide and sneak, dodge and infiltrate for specific types of combat mission, outside the principal battlefield.
One might argue that in the era of tricorders, a phaser-wielding combat formation has no use for stealth because sensors have such a vast lead in the rat race. The formation is best used by emphasizing its firepower and mobility, which brings back the issue of communication; brightly colored soldiers would be no better or worse targets than jungle-colored ones to the targeting systems of the enemy phasers, but they would fight more coherently in their role of walking (and sometimes transporting) artillery.
We have never seen personal shields in action, but we have heard of them. Perhaps they are a standard element in the type of ground combat we never really saw in DS9, the one that was taking place outside when Jake Sisko huddled in the field hospital in "Nor the Battle..". Troops utilizing personal shields would have no capacity for stealth (their shields would glow like lightbulbs on enemy sensors), and no use for it, either (their best bet would be to attack, not to hide or dodge).
The mechanisms and rationales would be many and complex, and far from intuitive for us poor watchers who are fixated on ideas stemming from today's combat. But one thing would be sure: future combat, using weapons as exotic as Trek does, would not be like today's combat. It might have analogues in some ancient types of combat, much like Trek space combat resembles fighting with sailing ships, or it might be completely unrecognizable. But it should never be expected to rely on rules that are familiar to us from the real world of today.
Timo Saloniemi