This Marine epoxy that is gray sounds odd to me. 20/1 mix ratio is very unusual. Are you adjusting this yourself or is that what the instructions say to do? It is essential that the resin and hardener be used at the ratio it is designed for. If any portion of the solid has unreacted agents within it the service life will be very short. the unreacted portion will eventually blush out soaking into the wood and softening it.
Dry pigments work well though I find the colors to intense. I have made two small batches of epoxy and filled them with two different colors of wood powder to a dough consistency. swirled the two batches together made the repair. after sanding I got a grain like appearance under the finish.
