Green, Yellow, Red
No it is not a stop light signal.
Depending on your add-on of choice (Grid, Healbot, VuhDo) your colors may vary, indicating levels of health.
In a perfect setting, one should be able to heal everyone back up to that green level….well what if that was not a viable option to you anymore?
Like many others have written, healing in Cata will be more of “triage” method of healing.
Our almost endless pools of mana in WoTLK will no longer be endless, and over-flowing. 
Instead of seeing a nice verdant plane of green boxes…we may see more of this instead….
not pretty I know. Healers are going to have prioritize which heal to use and when to use it. In addition, to who will be healed and who may die……
1.) Obviously – keep your tanks alive
2.) Yourself – dead healers don’t heal
3.) Other healers – see #2
4.) DPS
Don’t get me wrong there maybe times when you will need to heal a DPS over another healer possibly. But remember don’t waste the mana on the DPS who will not play along…you know the type! 
To prepare for this shift in healing, what can you do now?
1.) Get used to the idea….people will not always be sitting at full health.
2.) Get familiar will all the spells at hand. Also when to use what, when.
3.) Practice canceling casts as well. (we may need to do this, to conserve mana)
