Add the softened butter to a mixing bowl and stir until smooth. Add the lemon and lime zest and sprinkle with salt and pepper. Blend all the ingredients until smooth and completely mixed.
Place a sheet of plastic wrap out on your workspace. Spread the butter across the plastic wrap and roll into a log, twisting the ends to seal. Place in the refrigerator for 30 minutes to chill. This will give you a nice clean round of butter to top the fish with. Alternatively, you can leave the butter in a small bowl and dollop the butter on top of the fish once it's grilled.
Preheat your grill to medium-high heat.
Sprinkle the salmon on both sides with salt and pepper. Oil the grill grates with a clean tea towel. Grill the salmon for 4 minutes, and then flip and cook the second side for another 4 to 5 minutes. Transfer to a platter. Top each fillet with a slice of the citrus butter while the fish is still hot so it melts right over the top.