Most petstore bettas have a whole slew of different genetics and possible colorations in them, so when you breed them you really don't know what the fry will look like.
However, since lighter colors are the base coloration and typically don't show if there are other darker colored genes in there, I'd say that you have a pretty good chance of getting all light colored fry. If you were to breed your light colored boy (is he a cellophane?) with a dark female (say red or blue) then you'd probably get mostly darker fry because the dark coloration added would cover the lighter coloration...
is a great link (it's easy to understand and follow- very well written) on coloration and genetics if you're interested.