Change color post by Category

Can someone help me?

So this is the problem: I’m trying to change the color of the post when you hover the post (in a grid, like metro style), but the color change by category.

I already tried something like

global $post;

if(is_single($post->ID)):
				$class[] = 'post';
			else:
				$category = get_the_category($post->ID);

				if($category[number]):
					$class[] = 'bg-class';

but doesnt work.

also, I’m sorry but I’m new with php.