When to choose what colors I will play and vary my picks accordingly?
I think the question you should be asking is not "when to choose what colours I will play", but rather "when to stop choosing what colours I will play"?

Magic, and especially Magic draft, is a game that favours the adaptable.  When you make a first pick of a bomb in one colour, that does not commit you to playing that colour, but it suggests that you should weight all your subsequent picks accordingly.  If after the first two picks, your colour has completely dried up, then you need to be adaptable enough to let go of that colour.

This is not to say that, on the 10th pick of pack 3, you should still be open to moving into a new colour!  Hopefully your colours and the basic nature of your deck (aggro/control/mill/whatever) will be more or less settled long before then.  So how do you determine when to start taking cards that fit into your deck (as you currently envision it), as opposed to more powerful picks?

Unfortunately there is no quick and easy formula.  But I would say it's when you can actually visualise the deck you're going to play.  Let's say your first two picks are white bombs: lucky for you, but you still can't visualise a deck on the back of two cards!  Let's say white dries up thereafter.  Now you have to visualise a new deck in the colour that you're actually being passed.  Can you visualise it with a white splash?  You should certainly be trying, but not so hard that you end up shoehorning in cards that don't actually fit due to your attachment issues.

At some point you will have double figures of cards that you can see working and playing well in the same deck together.  It's at about this point that I would suggest that you should be committing.  With half a deck already now you'll need to consider optimising your curve and filling any gaps in your deck (too few creatures for instance). 

I'm always wary of people who stubbornly try to force a deck that they've visualised before they have a good number of the cards for it.  Sometimes it can really pay off, but passing early bombs in pursuit of a deck that may or may not come, well, that's the drafting equivalent of refusing to mulligan a no-land-hand in the hope that all of your draws will be perfect, isn't it?