Probably the bumpers strips are not in the color of the car, but in some kind of grey painted (can't see it very good on yor pic)? At least that is the standard for the Dynamique model.
In that case you can order a special spray paint at your Renault dealer for this, they use it themselves in the garage for fixing damaged cars. It's especially for painting plastic parts in the color grey of the Dynamique model.
I've used it to respray the grey plastic door parts (same color as the bumper parts) and the are now brand new again.
If your repainting them, you should take them of the door, use very light sanding paper and then repaint the whole part to avoid colour difference when paiting only a small part.
Order number at Renault is 77.11.238.903 and it says "BB Pierre 205139". Costs €19,-
Good luck!

Megane II 5D-Hatch 2003 * 2.0 16V Autom. Dynamique * Gris Boreal
17" Steppe * Leather * RS front grilles
Xenon + 5LED side lights * Clear side indicators * Ph. 2 rear lights
Cabasse Tunerlist * Pioneer TS-Q131C/TS-E1396 * Ice>Link Plus + iPod