Or tinted silicone caulk. But you have some other issues. The shoes for the balusters aren't aligned correctly. Only one shown is straight. Is the color of the sill what is intended for the finished look? If you intend to paint or stain it now someone has to finish the sill by hand, going carefully around all the shoes. Kind of painful.

Seam sealer. Comes in different colors. You can find it where they sell countertops.

It looks like there is more work to be performed. What does the builder recommend and/or intend?

That gap looks large enough to rip another strip and fill. Rip off the bottom of the female and it should drop right in. Stick it with a few dabs of silicone.

We got a wood worker to cut some red oak shoe rail to fit in the gap and it looks great!!