Can you discover the insect sitting on a leaf at the bottom of the picture? It's a rose chafer beetle (cetonia auratoa). You can tell by the distinctive white stripes on its underbelly and barbs on its legs. Vincent van Gogh had sketched the beetle earlier and probably traced over this sketch when making this painting. The beetle is painted far more precisely than the rest of the picture.
The colours in the painting are faded. Originally, the roses where not white but pink. If you take a close look you can still see a hint of pink here and there. The colour is fading because of the gradual bleaching of a pigment that Van Gogh used in the paint.