Travatan has lowered eye pressure in glaucoma 40% but I'm very afraid of possible eye color change. Could cosopt also lower pressure 40%?

Do not worry. Travatan is very effective at lowering pressure with almost no side effects, but more importantly can help prevent vision loss from glaucoma by lowering the pressure. The eye color change is rare, and might occur with long term use in people with blue eyes and a hazel center. The hazel part might appear darker or larger over a long period of time. Cosopt is ok, but has more side effects.
consider SLT. Additional information to dr. Chandler's excellent answer: many patients do very well with selective laser trabeculoplasty (slt) as a first line treatment instead of glaucoma eye drops. The laser works for you 24/7, you can't forget to take it, it won't change your eye color. Slt is repeatable when it wears off in a year or two. In my practice, slt has been safer and more effective than eye drops.