This may be sacrilegious to some, but I give my paper a quick coating of gesso to render it 'waterproof'.
I do not mind some visible light brushstrokes since they offer some interesting texture. I will use a smooth Bristol or cold press watercolor paper (and gesso on both sides to reduce warping). Each paper choice results in very different outcomes.
Once in a while I will tint the gesso for a base color. Next I paint with regular watercolors (not acrylics).
Since the paper is waterproof the paint stays wet for quite a long time. I often add other colors to the still wet initial wash to get interesting transitions and blends. Also, after the paint has dried, I might wipe some color off with a paper towel revealing that the gesso has been stained. This can be useful as an underpainting effect allowing for some semi transparent layering.
So, to a great extent, with this approach the coloring remains quite flexible. This helps with my next method.
It is hard to really describe what happens here since the results are often quite (nicely) unexpected. I use 90% rubbing alcohol on my brush. Just as an example, I will paint an area with fairly wet Payne's Grey then lightly paint into that a bit of bright orange. Not much happens. However when I then draw my brush dipped in alcohol across the two colors, interesting things start to happen…The colors separate and blend while developing some unusual textures. Much of this is initially a matter of experimenting to get the various effects where needed. Some colors work better than others. After much practicing the results become more predictable and 'useful'.
Attached are some examples (you can see more on my website - WATERCOLORS). In some the effect is very subtle while in others it becomes quite obvious. I need to add that all my paintings come from my imagination. This means (as with my oil paintings) the paintings evolve from no initial direction/objective.