How to do rollover images in Spirit theme

Hi - I have two images on the home page of my site that have links. The images are two logos, for Agathis and for Sure+ that are in grey. When you roll over these links I would like them to appear in colour - how do I create rollover images in the Spirit theme? the editing interface is very simple, but I think misses some of the more sophisticated features of the more basic standard Wordpress themes (unless I'm missing something)