I love how the little ghost sprite looks so far, it's simple but it's silhouette is still distinctive and charming! I think you're doing great on that front :>
As for colours, I think you're off to a great start there, too. Colour theory can get pretty complex AND pretty subjective, it always changes based on individual taste and the artist's intent. Here's a couple of links about harmonizing colour palettes you might find useful! I find that that's a good starting place for colour theory stuff.