It looks like you're trimming it too short. Let the whole beard grow longer and keep it at that length (half an inch at least, maybe a little longer.) Eventually the beard will become darker and fuller. It'll take a while though so be patient. After that trim it however you want. Try taking a biotin supplement everyday. It's supposed to make hair grow faster.
It's like when you shave your head. At first when the hair is growing out it looks kind of spiky and more blondish looking (if your hair is light/medium brown). But the longer it gets, the darker and more full it looks.