Grey is a fairly neutral shade that can work with a variety of colours and styles, whether you want to be formal or casual. As such, a pair of grey trousers can be a valuable and versatile addition to your wardrobe. Still, you do not want your outfits to become dull or repetitive, so you still want to change up the way you wear your grey trousers.
Navy is a particularly classic colour to wear with grey, though deep red can also work. You could also try a check pattern, though not too bold. Any of these combinations will give you a nice balance of smart and stylish, whilst black will take you over the edge into formal. Depending on your style of shirt, you could try an unstructured blazer or a chore jacket with your trousers, or even a leather bomber if you are heading out …