If you are sure this is what you want to do...
You need a good plastic primer and interior paint. Automotive interior paint is a bit limited for colours but plastikote will work on harder plastics. (It can flake on soft/flexible plastic)
PREPARATION!!!!
Clean...and then when you are really sure you want to continue.... mask and key the plastic with 1200 wet n dry. (You could use a chemical key, but if you get too enthusiastic you'll end up with holes in your dash

)
Vacuum car and cover everything with plastic sheets, to both protect from overspray and reduce dust. Mask off anything not already done.
Primer, 1 or 2 THIN coats, 1200 wet n dry followed by another couple of thin coats and 1200 wet n dry. Repeat until happy.
Same with top coat, 1200 wet n dry after every 2-3 coats until happy with colour density and finish. Clear lacquer to finish for perfection. (Same process as before) followed a week later by T cut and wax.
The above, although it is as much of a pain to do as it sounds, will give you an excellent professional finish. You can 'just spray on a couple of coats', but you will be reminded of that every time you get in the car.
Buy a 99p plastic car from Woolies and practice first if you are not sure, it is easy, just not quick to do.
oh and don't forget to send in some pics