String of pearls turning dark

Why are some (most) of the pearls slowly turning darker? It doesn’t look like healthy dark green color. More like dark grey-ish green. And it’s only the side that’s facing up. I have lifted some of them to check and the bottom seems to still be green. Some of them have also gotten some weird orange tint to them. I have them under a grow light and in a well draining mix. I may have been underwatering them a bit, but I doubt underwatering could cause something like this? I’ll try to water them more often but I’m worried that’s not the issue.