Logo in footer full colour on hover.

Hi Guys/Girls,

I'm wondering if anybody can help me with an effect I've seen a fair bit but I'm having trouble replicating. In the footer of all sites I create, I'm wanting to have my logo in grayscale, but on hover it goes full colour. (The logo image [png/svg] must also be a link)
I've found a few ways to do this in posts/pages or across the site as a whole, but I'm having trouble putting it in the footer of sites.
I don't mind whether I use a CSS effect to grayscale, then remove that effect on hover, or to reveal a separate image (have one grayscale, one full colour image).

I hope I've made sense, the effect I'm after seems so simple in my head but it's tricky to explain and implement.

Thanks in advance,

Jonny