The style of my game has the black outlines, a lot, so that's why they're so many 
Why does the vase need more contrast? then It'd look a bit gray, instead of white?
Naaaah, you can trick the brain more than that, bro. The power of contrast works on a bigger scale than that. Just cause something has gray shades in it won't make it look gray. And if it does, it's probabl because the bricks surrouding it are too bright. Which, incidentally, they probably are.
' 3 ' you should read up on this, and contrast between colors, et cetera, how it all works, this way you'll soon be able to paint a whole scene in orange and still make the human brain register a yellow-orangeish vase as 'white'. ' U ' the best artists have mastered this, so learn it, don't question it.