Sounds like he was just sleeping and was still "tired" when he first "woke up".
The light coloring sounds normal, a lot of bettas have it... it happens a lot when the belly (the area right below and behind the gills) is extended from food or growth. As long as it isn't white and fuzzy, or spreads over his whole body you don't have to worry about it.
Signs you will want to look out for in an ill betta is:
Lethargy- more then normal, rarely is active.. but don't confuse it with a sleeping betta, or if there is little decoration in the tank and no mental stimulation due to that.
Not eating- they may skip a meal once in a while on their own, but if he refuses to eat for days on end then you will have to look into the reasons and options to help him eat.
Swimming sideways or tilting- it's okay if when he stops he slightly tilts up head first, but not okay if he is completely vertical or can't stay upright and tilts to the side; trouble swimming.
He loses colors- color changes happens often as they age, and when you bring them home they will sometimes change in ways of brighten/darken/vibrant. But if he loses coloration and becomes pale then that is a sign of illness.
Those are just a few early signs of illness.. what you describe seems he is fine- bettas will sleep on the bottom or on a broad leaf at the surface of the tank. He may have been a little groggy- to wake him up gently, turn on the room light for a bit before the tank light. Move around the room like normal to catch his attention and once you see him swim around you can turn on the light to not startle him and give him time to wake up.