Well, more than painting the sandals, I think that dipping them in a vat (XD OK, more like a small bucket) of dye would work better...never tried it, but it seems like it´d work better than paint, since if the color took (and straw seems like a materal that would really absorb dye) it wouldn´t chip off... but then they´d still be lacking a bit of heels...maybe you could glue a heel, made of cork or some sort of wood (or plastic? o.O) onto the sandal? They don´t really have much of a noticeable heel anyway (or so it seems fom the pictures), so you might be able to forgo the heel altogether if you just stuck to modifying the color of the sandals....
I have never really had to modify a shoe for a cosplay....I guess I´ve been lucky so far... Good luck!