Automatic Color balance color: How exactly ?

If you want balancing handled on import, there is a preference setting:

If you would like to balance a clip before it is applied to any storyline, you can select the clip in the event browser and go to Clip > Open Clip. This will open it in its own timeline and any effects you apply to it in this state will always be used with the clip when adding it to a storyline (plus, you will not even *see* the effects you added to the clip in the storyline version — if you need to modify the effects, simply Open Clip from the storyline and adjust effects settings.)

Personally, I don't care much for the Balance Color tool in FCPX... I think it's a little on the "weak" side. Examples:

I used the White Balance option and selected the tarmac for the neutral color — and selected the same area in the custom example.

Another example for skin tones (custom balancer name obscured):

[the custom color balance effect has tools to modify the lighting and the "midpoint" of the range selection]

and for more severe examples:

both "samples" taken from the turntable platter.

I find the applied balance can be pretty random and that if you’re going to use that tool be aware of its limitations. One problem is that when you use different sections of a clip, let’s say a CU of a face, in the timeline you can get quite different results, which really isn’t what you want, when you really want the shots to be consistent. With balance it’s really better of you make your own selection. Also, the application has many other excellent tools for refining the color your clips rather than this one.

FCP will not decide for you what the best frame is; you have to choose the frame you want to balance the shot against.