With all the composite parts being black you cant really say the stock grips don't match the marker but I completely understand your desire to personalize your marker and I'm sure Planet Eclipse would give you that option if they deem it financially viable for them.
It's exterior caulking for windows and doors. It's flexible so it stretches when removing the front grip and still stays in place. I first tried this with silicone, it worked fine but had a shine I didn't like. Also, the silicone attracted dirt and dust. I'm very happy with the way it turned out.