Usually dark coloration is a good thing. When betta fish become pale, it can often be associate with something wrong with the water (although I also know that when betta fish sleep, their colour tends to fade.)
A good indicator that something's wrong would to look at his gills and see if he seems to be rapidly/heavily/having troubles breathing. If they are, there might be something wrong with the water.
If you just put him in the 1.5 gallon with the filter without letting the tank cycle first, he might be experiencing the ammonia/nitrite/nitrate changes in the water that occur during cycling process
. Ammonia and Nitrite are very poisonous to fish, so he may be getting stressed out with the fluctuations in the water.
I'd recommend investing in Nutrafin Cycle, it's a product that contains beneficial bacteria that you add straight to your filter and supposedly cycles your tank on the spot. I always let it sit for an hour or two so that all the water is treated before I put the fish in to be safe.