pigmenting epoxy

Yes, you can get specific pigments for it, but (within reason) almost anything can be used ... this chap does lots of experimentation

There are 'clear' epoxy adhesives but I think they still have a slightly yellowish tint, especially as they age. The kind of epoxy used for resin pouring does seem to have a clearer look to it - not sure if that's a different chemistry (maybe thinner so less air bubbles?).

For darkening it, surely you can add any pigment you feel like? Artists pigments are available in many colours, or just something simple like carbon black.

Theo

I used to make convincing copies of unobtanium brown/black bakelite knobs for old radios by taking a mould of an original and casting epoxy with ground up asbestos-free bakelite scrap mixed in the resin part

Interesting, thanks. I found some artists charcoal sticks in a drawer and ground one up a bit. Worked a treat. TW