I want to remove my sink in the bedroom permanently, but the water lines coming from the wall are not copper pipe. They are like plastic type of material, color is gray. Would I be able to cap these lines?

Most likely you have PEX pipes. These types of pipe can usually be capped using a SHARKBITE or GATORBITE PUSH TYPE cap... sold at all home supply stores.

These caps will usually fit copper, cpvc and PEX.

MArk