I have learned a few things whilst experimenting with Suminagashi; one is that it is quite addictive. If you try this you will need plenty of paper, I have found that watercolour paper works well, as does a mixed media card- like paper, even copy paper will work. The best results are made with handmade paper. You also need plenty of kitchen roll in order to damp off the paper.
By leaving the old marks on the water there will be an interesting ghost print on the next piece of paper.
It is fine to over print and it gives interesting depth to the papers. The original print is not disturbed. Also the other side of the paper can be printed without any effect on the original side, mostly mine are double sided.
Most of the time there is little control, at least whilst one is experimenting, but there are some marks which one learns how to repeat.
If you want to have a go at this you could join up to Creative Bug, the workshop is excellent, but if not there are some good videos on YouTube.
If you want to know how to get these pretty colours I can tell you, its a cheat, I used the original photo in a water colour app on the Ipad.