The most probable development for future camouflage (for all combat elements, not just mechs) would be metamaterials. Metamaterials are structured in such a way as to deflect waves around then in non classical patterns. There are actually many different ways of doing this, but the optical lattice is the most commonly used and understood.
Essentially, an optical lattice is a structure with gaps that are a fraction of the size of the wavelength that you want to manipulate, with the gaps engineered to "bend" the light in the direction you want it to go. In theory (and now in practice) you can bend the incoming wavefront to "flow" around the object being protected so there is no reflection or refraction to indicate the object is even there. Since the waves flow around it, there is no "hole" to indicate its presence either.
Current metamaterials work in the electromagnetic spectrum (radar and microwaves), and in limited optical frequencies (usually one or a small number of frequencies). Metamaterials can also be scaled to deflect sound waves; reducing the ability of sonar to track ships and submarines. Very long wavelengths, like the ground waves of earthquakes and explosions could also be deflected around buildings and structures: a metamaterial shield on that scale would look like a grid of pilings driven into the ground around the protected structure.
Metamaterials are passive, which means the operator doesn't need to monitor or activate anything, but based on their nature, they will require lots of care and cleaning to remain functional, so a tank or armoured vehicle bashing through the woods or in urban combat could reveal itself as the covering becomes damaged.
Metamaterials can be used for other purposes. Bending waves can be used to increase the sensitivity of antenna and optical components of cameras and sensors by focusing the incoming waves to the receiver. Laser and microwave emitters could also be vastly improved by using metamaterials rather than lenses to focus the beam.
Metamaterials are not a cure all, however. The heat energy of the vehicle or soldier still needs to be managed somehow, and the use of energy (sending a radio transmission or using a weapon) will also reveal that something is there. The vibration of a moving vehicle can be picked up by geophones, and metamaterials themselves are designed around specific frequencies: an optical metamaterial shield would have little effect on radar, while submarines sheathed in sonar cloaks would still be visible on the surface. And vehicles in space would be revealed by their heat signature against the cold backdrop. Targeting in an age where metamaterials are common would involve large numbers of multispectral sensors employed in a "grid" to identify anomalies and direct probing fire to reveal what is there. Area weapons like thermobarics, large bombs and fuel air explosives would also make a comeback to ensure that whatever was in the target grid was actually hit.