It is important to pay attention to what you wear for your engagement shoot in order to improve the quality of your final photographs.
Bright or even bold colors can very easily be a distraction, stealing the focus of the photo away from where it should be, on the couple and their faces, expressions and interaction with each other. The eye naturally goes to the brightest, boldest and most prominent part of a photograph. Its just how it is.
With this in mind, I recommend wearing pastel colors, particularly for the tops. Either matching or complimentary colors. If the match is a little off rather aim for complimentary colors, still keeping it as subtle as possible.
While this example is okay because the late afternoon sunlight softened the harsh white of the shirt, a light green or blue shirt would have been a better match. See how his white shirt draws your eye in first before going to the rest of the photo? This is what you are trying to avoid, anything that overpowers another part of the photo.
It is okay though to go bold to an extent, both in white shirt & dress for instance. Or black, dark grey and even up to a darkish blue. But avoid bright saturated colors like red, yellow, pink or orange, even green. This will just totally steal the show. Same counts for bright or contrasty check material.
If it suits your personality & theme, printed T's that match are also a good option. In that case, definitely matching colors. And remember to keep the correct positions if there is a phrase printed that runs from one shirt to the other, its a theme and a message that is better to keep in the correct order.
This session is from 2019.
Black shirts worked very well in this case. They are bold and draw attention to the couple while at the same time they also served to shift the attention to their faces despite the fairly bright patches of background.
The best advice I can give you is to discuss this with your photographer and take location of your shoot into consideration as well as the time of day and expected direction of light. These are very important factors that will help your photographer plan better and result in better images in the end.