Where/how does one get gel coat color matched? My deck is off white.

If it's relatively newer, you can call the manufacturer and have them send you what you need to fix things.

If not, then you have to get pigments and try to tint white gel to match. White is a lot easier than colors to tint to match.

Who is good to use. I am removing my bow rail and need to fill the holes.

Just as above. Keep in mind, white becomes "off white" using the entire spectrum. Typically, yellow.....but bits of black to take away the glare.

It's much easier using the "CMYK" code if.....IF the builder will provide you that info...

What is the accent color of the boat?

It will look good and give you space if you put a hardline over the repair area once filled and faired. Plus, it won't look like a bandaid....