buy temple white. that's a liquid that can be applied to your own hair (with a paintbrush or toothbrush) to make it look greyish or whitish and will wash out easily. it's not as messy as hair spray. kryolan makes this stuff, so you'll get it at a store (online or offline) that sells theatrical makeup and special effects makeup.
the best result starts with a good wig though. for the future, try to find a wig that's your size (actually, it really helps to just pull down the wig with force and wear a wig cap so you can keep your own hair covered and thoroughly pin the wig to the cap with hairpins or bobby pins.) plus, if you know you're having a problem with your own hair showing, try to style the wig in a way so it covers your hairline - don't cut it quite as short, and leave some loose strands in the back of your neck and on your temples that hang down over you own hairline.