I keep reading it on many reviews that you can extract colour from the enemies you've defeated...
Is it possible? HOW? Because i dont see a reason to kill enemies if not to extract color from them. (expect some cases)

Another thing, is there a way to kill an enemie more quickly?
Putting the amount of crimson in you aside, is better to focus the color on one dot, or to draw a line or something like that??

I have not beat the whole game, but from what I have found, only certain creatures drop color when you beat them, and the color is automatically drawn towards you in the shape of a sphere. As for attacking enemies, I don't think it makes a difference whether you draw a line or a single dot. You do, however, gain attacking glyphs as you progress through the game which helps in cutting down the color expenditure a bit. Though, I usually make it a point to avoid most enemies, only fighting as a last resort; color is supposed to be precious, after all.

Correct. there are some enemies that have colour in them and can give you a marginal benefit. the big slow moving sacks with flappy arms (can't describe it any better really) tend to have colour in them. Also the big monsters like the Pangolin, Whale, Worm and all of the brothers will give you colour if you defeat them however you usually end up with a loss. With the brother you'll probably get a large loss since it costs a ton of colour to make them kick the bucket.

Crimson helps though and using attack glyphs is more effective but against small fry the best advice is avoid them if need be or just drop some colour on them as some glyphs can miss which again makes them costly.

The big floppy Predators that suck you towards them when they see you, as has been mentioned, drop about as much color as it takes to defeat them. There are two kinds of Predators that appear in damaged realms that also drop color, but usually less than it takes to defeat them.

There is only one slight advantage to killing the big floppy Predators, and that's that they can drop any color (though, in my game they nearly always dropped silver), which means you could spend a color you have a lot of to maybe get some you have only a little of. In my last game I managed to get silver several cycles before it appeared in the Void, so I imagine in the very early game when you first enter Heathen you could gain a slight advantage because there are several colors that won't have appeared yet that you can get from these and then plant.