It depends simply on the position and the centre.
what color bishop (light or dark), is more important for both white and black ?

It depends simply on the position and the centre.
is any general rule of importance applicable, if either light or dark bishop has to be choosen, when the game condition is equal ?
thanks.

There is no general rule for what you ask. It simply depends on the current position, and to a lesser exent on the opening played.

In many openings Black has trouble getting good use out of his light bishop, but to be honest you have to be playing to a fair standard before this matters. Both sides King's Bishops (White's light and Black's dark) are on the same colour that the enemy King will go to when he castles, which can make the KB a valuable attacking weapon -- supposedly Tarrasch used to swear by it. But so far as that goes, see the first sentence again.

The problem piece for me is always the light sqaure bishop with black. So as black I am happy to exchange it (but its hard to force exchange). But it is only due to the opening I like to play.
hi friends!
1) what color bishop (light squared or dark squared), is more important for both white and black ? (both sides are equal).
i.e. a) what should white retain among light squared or dark squared bishop ?
b) similarly, what should black retain among light squared or dark squared bishop ?
please give your opinion and justification.
thanks.