Brown bottles are not needed! Clear glass or plastic which prevents air from entering are OK. There are many threads on this subject here on APUG. Either fill the bottles to the top, use marbles or use Nitrogen.

Your biggest problem is mixing powder with water. This involves a lot of air. Liquid kits involve more air entrainment. I suggest that you look into them. A larger liquid kit with less air is probably better than a solid powder kit with more air during mixing. IDK for sure.

So, I mix, fill to the top, screw shut and done! In fact, some bottles have concave tops to force tight fits.