Here is why the pants are turning Tan. Note, I'm on PC, but even at Skyhold my outfit would get 'tanned' for no reason. (I played with mods my 2nd/3rd time around) This is because of a little thing called Dirt Buildup. This is a feature in the game where depending on where you walk, dirt will build up on your armor/clothing like it would in the real world. Sadly, for those on Xbox/PS4, you cannot use the mod that disables this, but the 'glitch' is but a feature in the game, from 3 play-throughs of experience, that can be annoying. Your game isn't broken, 'tis only an annoyance that Bioware has placed in the game.
Again, I'm a PC player, but as far as I know, this is NOT a glitch! Take this with a small grain of salt.