Target specific thumbnail on homepage CSS

Hi, I would like to target the specific thumbnails on my homepage but cannot work out how to find what the individual elements are called using CSS. E.g. At the moment the background of both the first two thumbnails are yellow. How would I target the first one so that it has a red background, and the second has a yellow background?

Thanks

Liv

My site is: greyhound-keyboard-sa3k.squarespace.com

@LivAils Add to Home > Design > Custom CSS

#projectThumbs a[href="/londonisopen/"] img {
    background: #ff0000;
}
#projectThumbs a[href="/summer-of-sport/"] img {
    background: yellow;
}

Amazing! Thank you so much

@LivAils You can Like & Upvote if it is correct. Thanks.