A betta's poo is usually the color of whatever he ate (also usually the shape too). If he ate a brown pellet, he will likely pass a brown pellet shape poo. If he ate a bloodworm, it will be a darker red thin poo.
And yes, I agree with Pitluvs. A full 10 days is probably a good way to combat SBD and make the chances of recurrence slimmer. I've done the same thing, putting my HM in epsom salt for constipation and taking him out after a few days because it seems to be working. But then he just gets constipated again. 10 days in epsom salt won't hurt him at all, although it will make a little more work for you.
And don't worry, I've asked lots of poo-related questions before. Poo, uh, actually seems to play a big role in betta care.