I use Phososhop, generally, but I occasionally pull out Manga Studio/Clip Studio 5. I definitely recommend it - it’s pretty different from Photoshop, true, but it’s faaaaar cheaper, and can do some pretty nifty stuff. That’s what I use for my faux watercolor things!

As far as tutorials go…I don’t have anything specific? A lot of it is just messing around with whatever tools are at your disposal and seeing what works for you. If you have a bit of money to spare, Kyle Webster’s impressively large collection of Photoshop brushes are fantastic (the gouache brushes especially are 1000000/10), but the tools PS comes with already are nothing to scoff at, especially if you customize them to fit your tastes. (Also there’s free brushes everywhere.) On the Manga Studio side of things, I don’t have anything specific to recommend beyond the same mess around + customize your brushes thing (it’s always worth looking around, though! I’ve found some pretty good free brushes for MS on deviantart), but I did adapt my faux watercolor technique from this tutorial for Paint Tool Sai.

Thank you so much for asking! I hope this helped. : )