Thank you for your comments Joy, Cindy, Tanja, Kay and Claudia.
Claudia, yes the paint is sliding of the yupo easily, I experienced the same but kept trying.
I did not see a big color shift during the drying, but the drying times were extremely long. (OK not for an oil painter, but for a watercolor)
Kay, do you wipe the yupo with alcohol or something like that before you paint on it? Maybe the color would stick better then.
But I haven't tried it yet.
Claudia, your persimmons look beautiful, the color of the fabric is a bit dull, but that makes the persimmons stand out better.
The dull look could be a result of the brand you use. When I want colors to get really bold I take sometimes QOR colors, they can be mixed with any other watercolors but their binder is more transparent, so less color shift.