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 on the town.
In the winter, a grey roll neck can be a practical and stylish companion to your grey trousers and navy jacket. With your grey trousers providing a note of formality, a less formal pastel sweatshirt will not look out of place if you want to avoid a full jumper. In some cases, a cardigan can bridge the gap between jumper and blazer.
In the summer, a Cuban collar shirt can provide a cool and trendy addition to your outfit. As you are wearing plain trousers, a floral print shirt will not look out of place, but you could also choose block colours. It can be worn on a casual occasion, but adding a blazer can make it smart enough for a summer wedding.
You can wear any colour with grey, but other options include a plain white oxford shirt (perhaps with a simple tie) or a less restrictive but equally smart white collarless grandad shirt. You could also tuck in a plain t-shirt under your chore jacket. A look at the Farah shirts at https://www.ejmenswear.com/men/farah shows the range of styles available.
If you are dressing more casually, trainers (preferably white and with a low enough ankle to show off your socks) or lace up boots are more than suitable, whilst a more formal affair could involve anything from penny loafers to Derby shoes (as long as they are polished).