If these are web colours, and you're entering them into the Terragen colour picker, then you should leave the gamma correction at 2.2. The issue is probably not the gamma, but the exposure (or sunlight intensity). Terragen's default scene is exposed quite brightly to work well with landscapes and vegetation that are typically low albedo (ability to reflect light, including diffuse reflection). If these web colours get up into the 200s (about #C0 in hex)then they will be massively over exposed. If they do, then I wouldn't trust them to represent realistic albedos.
Matt