Soloing yellows and oranges isn't tough, quite easy actually. Yes, good armour is necessary, keeping your spells as updated as you can (finding them being the hardest part, I still haven't found 2/3rd of my adept spells, or more), but solo mobs are easy.
I got to lvl 26 soloing, fairly slow as I don't play that often, but I only group for mobs that I know I can't take alone and quests require them.
Fighting heroic blue mobs, that's just never a good idea. Solo mobs might not give the best of xp, but combine them with quests and xp should come quick enough.