If its Polyester, the red or blue harder carries a dye so you can tell how well you have mixed the stuff.
Gelcoat is polyester as well, and uses Methyl Ethyl Ketone Peroxide (MEKP) as a catylyst.
Its a clear liquid. Keep it out of your eyes!
That will also work in your Filler. Too much and goes off too fast. Experiment.

Cheers
Mark

On Wednesday, February 17, 2021 at 12:15:47 PM UTC+10, AS wrote:
On Tuesday, February 16, 2021 at 2:08:38 PM UTC-5, Roy B. wrote:
Does anybody know a source for light colored 2-part polyester putty? The ones up here all have a bright colored hardener which helps on the mixing but shows through after a thin coat of white paint. I'd like something in a light grey or white color (after mixing) and preferably in a small quantity.
Thanks
ROY
Roy - try 'UPOL Dolphin One Fill UPO660'. It is a white 'All in One Premium Body Filler' (TiO2 pigmented) which comes with a white hardener paste - doesn't get more white than that. It says it won't clog sand paper and is resistant against pin-holes.

Uli
'AS'