I found this image on pinterest and I just love it! I would like to know if there is a way to reproduce this effect on Illustrator? It looks a little bit like watercolor, but I can't really tell how it was made.
In my opinion the easiest and quickest way would be mesh tool and just some desaturated/pastel colours.
First create a mesh'ed object. I already have opened desaturated swatch library (open swatch library > color properties > desaturated). You can also create your own or import one that suits you best.
Then, with direct selection tool select some points and add colors that you thinks look the best.
And play with points placements, add some curves. At first add only few point as you can add more later.
Recreating this in Illustrator is also possible without the use of the plugin Franz provides. While this plugin makes the whole process of a watercolour look a whole lot easier, you can still work without it.
What you still have to do in Illustrator is to define the bigger outlines of the 'waves' you see in the picture. Adding multiple of this areas, filled with gradients, is then key to get a similar effect, after continuously playing around with the opacity to get the desired look. But since doing this by hand takes a lot of time, you will need to use similar techniques as in this short tutorial, where they 'randomize' areas to work with and to generate a watercolour effect:
To get rid of the 'sharp lines' you will also have to play around with the feather, or the shape of the colours.
If you really look for watercolor effects and even to add some textures, you should have a look at the Texturino plugin, which lets you paint various effects on your vector shapes: http://astutegraphics.com/software/texturino/