Every player have a brain (I think ?!) and memorising the ennemis heroes shouldn't be a problem... for new players it's a little difficult to know who is who I agree, but after some games I'm sure they'll be ok. (like I did)
And about the color feature... I think it's already available (I didn't use it yet) on the minimap there is a little icon for that
To be fair when I started HoN it was difficult for me to point out enemies as well. I think the red line is too thin until you get used to it (when you hover the mouse over an enemy). Not sure how one would go around this, though.
I would like some kind of aura effect around heroes/creeps. This would make watching games much more comfortable to beginners.
If a normal gamer watches a tournament stream of CS or Starcraft they have usually no problem to track what unit is in Team A or Team B, but with games like warcraft it becomes hard, especially when many units are involved.
Atleast in replays or as spectator some option to change this would be appreciated.