My cousin wants me to custom paint his transformer toy, but it keeps smearing.
You may find to need to wash or even rough up the plastic texture a little so paint will stick to the plastic. A simple wash with an abrasive like comet or a light sanding with extra fine sandpaper will help. For larger areas or as a primer one could use one of the spray paints made to adhere to plastic like Krylon Fusion.
One thing to remember ever where the parts join and move against each other the paint has the chance to crack and or chip. To help avoid this you can sand, buff or whittle the area before parting to leave room for the newly painted parts to have room to move freely.