getting a paint to stick on a thermoplastic compound like polyethylene, or polypropylene is very difficult, and i will bet money that the fenders on your bros dirt bike is self coloured polyethylene, and no paint will bond to it, on on plastics like polyvinylcloride, or Vinyl paint and dye will be able to penetrate and bond quite well, yoot computer is peopably made from a bi-blend plastic containing Polyvinylclorice (PVC), polycarbonate (PC, or lexan) and is a good base for paints
and pzatchok: your brother should either get a refund on his college tuition or you just didnt pay attention when he explained to you that polyethylene and polypropylene both are made from the gasses, ethylene (or ethene) and Propene (the suffix -ene, instead of -ane refers to a double bond, meaing that it is unsaturated) this bond used to polymerise a long carbon chain, depending on the process you can either get High density or low density polyethylene, that are respectively relatively stiff, or relatively flexible. (the lids and bases if tupperware are HD-PE and LD-PE respectively) but both plastics are FDA aproved for food use and will under NO sircumstances EVER leave any taste at-all. period!
other plastic types are also polymerised chains of chemicals containing carbon, hydrogen and a host of other elements like clorine, suffur, bromine and many more. the possibilities are endless and the properties are as different. and to add to the confusion they can be blended and mixed with other plastics and solids or fibres, as an example, nylon can have peoperties like fishing line if used by it self. and mixed with glass fibrers be as stiff and strong as aluminium, the frame of a Glock pistol is nylon with glass fibre filling for example
and my knowledge of plastics is limited to 4 semesters courses at university when i was studying to ge an engineer. and ive only scratched the surface..