Thank you Mr. mikewa-ga. I understand what you are talking about. But,
i just want to know how and why blue and white colonies are produced.
Mr. mikewa-ga, just make a couple comments about this. Also say
somthing about plates. I just want to get a idea what is going on.
Please do this for me Mr. mikewa-ga, and i will certainly appreciate
it. All of this is under the lac-operon.

Xgal is 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside

It is metabolized by beta galactosidase releasing the indolyl moity -
the resulting product dimerizes forming an insoluble blue dye.

If a cell has functional beta galactosidase, Xgal will be metabolized.
The cell will be blue.

If the cell does not, then Xgal will not be metabolized, and the cell
will remain white.

When we are talking about plates, we are talking about a petri dish,
or a cluster plate or any surface plus growth medium on which the
cells are placed to grow.

You can plate cells on different media which have different nutrients,
antibiotics, etc...

I hope this will help you to understand. 

X-gal is used as an artificial substrate by beta-galactosidase, an
enzyme that converts it to a blue product. If a researcher grows
normal bacteria (which have a normal beta-galactosidase gene) in the
presence of X-gal (added to the culture medium), they multiply and
grow to form a blue colony.

Mutant bacteria with a mutant (non-functional) beta-galactosidase gene
cannot make the blue product, and are therefore white.

You might want to see a practical application of this fact at:
http://www.edvotek.com/300.html