Mate, I think any fine wood sanding dust will do the job. Just what-ever colour you like. The Epoxy will stain it much darker than the bare timber,
I collect bags of it from my work, but most of it is an indonesian sub species of WRC - sirian. it goes very dark when mixed with empoxy and will stain bare ply badly. I try to protect my ply as much as I can with masking tape and a pre coat of clear epoxy before I start to stitch.
My first S&G build, a swampgirl turned out with dark brown/redish stains all over the ply from the stuff. It was a horrible mess.