I’m finishing pine with amber dewaxed shellac (with a brush). I was hoping it would impart to the pieces the same color as the flakes after a few coats, but it’s still looking too yellow for my taste after 4-5 coats.
FFR this is a general rule for shellac, the flakes are always going to be darker than the finish looks on a lighter wood unless it is built up very heavily.
Is there anything I can add to the next shellac coats to make it darker?
Yes, a darker grade of shellac or an alcohol-based stain/dye*. There are numerous woodworking stains based on shellac but also leatherworking colours will work. These can be used to tint shellac as well as to colour wood directly.
It's worth picking up more than the one colour you think you need as it often takes a bit of this and a bit of that to get just the right colour. And remember to test on scrap before committing to the workpiece.
A word of caution using alcohol-based dyes: they're very concentrated so a little goes a long way!
I’ve read it’s possible to obtain very dark shellac by dissolving audio records (33s, 45s) from the 1940s, but that seems like a waste.
It does seem a waste but records old enough (78s) are still abundant in some places and any that are cracked, or badly chipped, are no longer playable so this is actually an ideal way to recycle them.
The only problem with this method is it produces black polish, and you won't generally get a pleasant colour by just adding black to a lighter colour of finish, generally you want to add a darker brown or something like that. Here, adding black might actually yield a greenish colour (sort of like olive drab)!
*You can also colour shellac with powdered pigments but this is trickier to do well and the resulting finish will mask the wood more than a dye-based colourant will, basically very much like oil-based stains and "gel stain" often do.