I know what that basically means, but I just don't know where the line between "too dominating" and "perfect" goes.

The laptop monitor is beyond fixing with tests like that. It's a four year old flat screen...

Colour balance, or colour composition is a more widely used word, is partly about making sure that one or more colour isn't too dominating or too subdued. If there are elements in the image that feel too prominent you may need to adjust the colour. General composition plays a big part as well of course, and I think your composition is good. But as I suspected the background dominates a bit too much when viewed on my good screen. Try to adjust the colours so you get less contrast. (I don't think you would get a good result if you just adjust the contrast of the image, I think you need to work with the actual colours.)

Most of my friends said that the biggest reason are they ears.

"It usually takes me a bunch of tries before I get a decent result." I'm not an artist. I don't even know what "balanced colours" exactly means!

Try to fix your screen with this one:
http://www.softpedia.com/get/Multimedia/Video/Other-VIDEO-Tools/Nokia-Monitor-Test.shtml

"And one funny thing more... All my friends have thought, that that dog is a cat!"

Hmm. I think that's a reasonable conclusion. Many of the features you and I know are there are hidden, making it kind of cat-like with the way the flames make the nose look like another shape with the way it highlights and all.

"I'm not so good with colours and everything that has something to do with them..."

I still have trouble with balancing colours. It usually takes me a bunch of tries before I get a decent result.

I'm not so good with colours and everything that has something to do with them...

"Why not? :)" I'm going to put a flaming room and that dog to Sample, but not them both in a same scene. I'm going to make a scene, where Sample, assistant and soldier team enter a big room, wich is totally in fire. Then, a flaming mutant (mutant style under remake) jumps out from the flames and attacks. That mutant is actually the other Sample, the first mutant in the lab, who is now invincible and wants to kill the other Sample because he is angry. Sample (that stickman one) and the others escape, but the other Sample returns in the final scenes when the bomb is set and they are getting out.

And one funny thing more... All my friends have thought, that that dog is a cat!

"It has almost nothing to do with Sample." Why not? :)

Nicely composed! But, I am on my laptop, which leads me to believe that you might need to tone down the colour intensity a bit. When it looks great on my laptop that is usually the case, it has a very sucky screen...