It's actually really easy to do, and practice makes perfect!
-First, use an epoxy mixture to fill in the imprints left by the brand's logo.
-Then, after that sets and dries (give it about 24hrs or so), use a "finer" grained sandpaper and lightly sand the surface of the areas you want painted.
-MAKE SURE TO SEAL OFF THE AREAS YOU DON'T WANT PAINTED! Use painters tape or something!-In this case, you'll be sanding the plastic armor parts. Remember...lightly sand it! You don't want to destroy the surface, just give it enough texture so the primer can grab hold of it.
-After you've sanded, pick out a primer from a good paint brand. I don't know the recommended brands for Europe, but I'm sure you can find a few. Primers are usually grey or white, but you can use black sometimes. Either way, give the areas you want to paint a couple of coats of primer,
letting each layer dry before you apply a new one. I usually give a good three to five layers of primer when I paint. This helps the paint stick to the surface as well. After you've primed the piece, you can sand again, but do so
VERY LIGHTLY (Though you might not need to do this, it does help sometimes).
-After you've primed what you want painted, it's time to paint! Pick out a color, any color! You can do different effects by layering paint differently, so I'd suggest maybe doing some online research and you might find something you like (such as some metallic looking armor or something). Either way, however you want your armor to look, you can figure that out. If you're going for a simple color (Black, silver, etc) you can do that as well. It's up to you from this point on. Do the same thing this time around that you did with the primer. A
few good layers of paint, letting each one dry after the other. Make sure to take your time, so that your layers aren't all clumpy and the paint layers smoothly.
-After your paint layers have dried and set completely (you can usually primer and paint in the same day, because it should dry quickly depending on the climate and humidity. I forgot to mention, make sure to try to do this in a fairly dry, safe environment with good ventilation. If you can't access a place like this, the outside world is also a good place to start. Make sure to have a tarp and stuff ready for a workspace, and keep your goods sheltered from any inclement weather!
ALSO, MAKE SURE TO WEAR A MASK IF YOU NEED TO, OR USE THE PROPER GEAR TO KEEP NOXIOUS FUMES AWAY.),you can apply a clear coat. Again, I don't know what brands they would suggest over in your region, but a clear coat will protect and seal the paint onto the armor for a fairly long amount of time (perfect for costuming and long-time use, years if you treat it well).
Apply your clear coat just like you would with paint, layering it gently and smoothly and letting it set/dry with every layer. -After your clear coat has dried completely, voila! One set of armor!
And just for reference:
http://www.instructables.com/id/Trash-to-Treasure-using-spraypaint-to-refinish-cu/step5/HOW-TO-PROPERLY-APPLY-SPRAYPAINT/http://www.krylon.com/how-to/safety/spray-painting-best-practices/