Picking out the recesses of fluting in a darker colour looks like a masking job. It is not, and it cannot be, because tape on a curved fluted drum leaves a wavering edge every time and one lifted corner ruins a whole column.
The line is cut, not masked. When the flutes are worked, a fine fillet is left standing at the top and bottom of each reed - a raised bead only a fraction of a millimetre proud, running the full length of the flute on both sides. That fillet is the boundary.
The darker colour goes on first, over the whole drum, and cures. The lighter taupe is then applied with a pad drawn along the reeds, so it takes the raised faces and stops dead at the fillet on either side. The fillet physically holds the pad off the recess. The result is a crisp line the full length of every flute, identical on all of them, because it was carved rather than drawn.
It also means the line cannot drift over time. A masked edge is only as good as the tape that day; a carved fillet is part of the timber, so if the piece is ever re-finished decades from now, the same boundary is still there for whoever does the work.