Bonjour!
I'm not clear if your grip is wood or plastic???
If it is plastic and you want to make it look darker without loosing the "wood grain" look, then one way is to apply a dark brown paint "wash".
A "wash" is a paint that has been thinned down (with paint thinner) so that the ratio of paint to thinner is only about 2% paint (or even less). You can apply the "wash", and let it dry; then if it is not dark enough you can apply another "wash"; and repeat the process until it is the dark colour you want.
It must dry thoroughly between "washes". Check on the internet for "paint wash for models" for more information about applying "wash".
If the grip is actually wood, then the wood stain mentioned before is the way to go.
A bientot, mon ami.
Kiwigunner