First off, I will never tell you NOT to wear something. If you are in love with a specific shirt/pair of shoes or just have sentimental value with something, we'll work with it- but you have to trust your photographer :) so lets dive in to what to wear to an engagement or couples session!
If you are choosing to get married in a forest, I always recommend complimenting the colors of the forest at the time, without MATCHING them. If it's fall time, wearing beiges, whites, blacks, khakis, and any neutral tones, make the photos beautiful! If it is summer, when the trees are still green, keeping the neutral tone idea, but adding darker tones can work really well too :)
If you are taking photos on the beach, incorporating lighter colors with a pop of color, or incorporating an array of light to dark neutrals can really enhance your photos. I always suggest complimenting your partner, if they are wearing greens and khakis, I don't suggest wearing reds and blues. The easiest way to coordinate is by sticking to the neutral or muted tones !
Most importantly though, you have to be comfortable, please wear shoes that you know you can walk in, and clothes that you feel good in. If you are uncomfortable, it will show in the photos. On top of being comfortable, these photos are an opportunity to represent you two as a couple! Be yourselves while choosing clothes to wear.