Graphics Gale has a *very* nice curve tool. It's a good pure pixel art program and you can make tiles with it by using the tile preview feature.
For color adjustments, post- and batch processing photoshop/gimp is best IMO.
Krita isn't great for pixel art but it makes very nice tiles. And while these aren't for pixel art either, I use Sai or Clip Studio Paint for stuff like portraits.