If they die at your feet then its good xp
Green is normally the way to go. There are 3 levels of green monsters though, one bellow you one at your level and one above you.
Find the monsters who give the best xp per kill and they are most likely above you, same goes for worst xp and green.
When hunting greens its not about best xp or worst its about how many you can kill and how fast you can kill them before taking a break.
If you rely on your skills alot this means you will most likely take out 3 for example at lvl 24 before needing to recharge your SP and HP.
Keep yourself buffed and attack things at FULL range. The more arrows that hit at range the more damage you do faster, because when the monsters are in your face you do LESS damage than at range.
Try to find monsters which wont debuff you. Killing blues would be acceptable if you are constantly killing them (no break).
Do the 10 minute marksman song test to see what you should kill.
Take a look at your xp and take a mental note of %, now cast your buffs, marksman song included, and kill till the marksman song wears off and look at xp again.
Now go do it with other mobs and compare, remember to keep killing as fast as you can and constantly for each test.
You might find you lvl faster killing certain mobs over another.
If you are a STR DEX build dont worry about attack order each time. Just keep pulling in the monsters till you need to replenish health poping off skills as you can use them.
Always use a DoT at range, even better first shot, always use pierce at range. Remember your missle debuff skill will give you + to each shot so use it.