one of the fascinating aps available on ipad is 'paper 53'
with it you can do pencil sketches and ink washes
watercolor and colored ink or felt pen (wide or narrow) - with a 'resist'.
and any mix of them all that you choose
the main difference between them is the texture and density of the 'stroke', and that some are transparent and some are not. the watercolor brush will also 'lift color in a way similar to a dry brush on wet paper, but it's much more (disappointingly) controlled and uniform than the real thing
the ap doesn't have the subtlety of real artists' washes and pen / pencil strokes, or the precision, or any of the lovely and miraculous surprises, but it does have one very attractive feature, which they call 'rewind'. with this you can remove the last stroke(s) or wash(es) or erasures you've done, without altering the picture. you just get what you had before you put the new bit in... then, if you decide you liked them better than you thought, you can change your mind again and put them back. there's no damage to the 'paper' no matter how many times you go over things, and it's very hard to make things muddy. no worries about whether the paper is wet or dry - which also, sadly, means there's no way (that i've discovered so far, at least) to paint wet-on-wet.