Almost all bettas change colour during their lives. It can be in response to environmental stressors, age, or illness. Usually, a light betta is stressed and a darker betta is more chilled (this isn't true for bettas that are naturally light, such as white or cambodian bettas). When I first got my girl Athena (RIP), she was pale brown with a lavender tail. When I got her into my 5 gallon tank with a heater and filter, she went a stunning royal blue with a black head and red streaks in her fins and tail.
Is it possible for you to post a picture of your boy? Also, can you list your tank stats (size, temperature, tankmates, water parameters etc)? Has his behaviour changed in any way? I'm betting it's just because he is happy in his home with you, but pictures and the extra information will confirm that. :)