Spectrum is your best bet for small repairs. It is important to make sure the screw holes are clean for the material to adhere strongly. he holes can be drilled out one size bigger to clean up the holes but the putty works best on small diameter holes ( less than 1/4 inch).
When using the putty, you can mix small amounts and although it sounds extreme, a digital scale will help get the right mix of putty to hardener. The small bottle holds two ounces and the instructions will tell you how many drops of MEK(P) hardener to use per ounce. Once opened, the jar of putty has a recommended shelf life of 2 months and unopened has a shelf life of 6 months although I have used previously opened jars stored for a year. Over time the putty gets dried out.
When filling holes, sand the surface a bit around the hole and apply the putty proud of the surface. In other words don't try to make the uncured putty perfectly flush. After curing, you will need to sand, compound perhaps and buff. It all depends on how perfect you want the patch to look
I think Spectrum Color still has a how to video on their site.
Regarding colors, the Spectrum colors match the original Grady colors very well but depending on factors like age, your gel coat may have changed hue a bit. A perfect match may be difficult to get but the larger the patch area, the easier it is to feather the edges and blend in the patch. Whether all that extra work is forth it, is up to you