PORT is a red wine, and there's always some red port (wine) left.

Explanation

This is a mnemonic to help you remember that PORT is the name for the left side of a boat or ship, and that the port-side navigation light is red. The trick is simply that PORT and LEFT are both four-letter words - and port wine is red, tying the color to the name. The first letter of each word gives you the first letter of what it stands for.