Two trays of marigolds can look almost identical while they are small and still make very different plants in a border. Read the expected mature height and spread before choosing by flower colour.
A compact plant at the front of a path has a different job from a taller plant among summer flowers. Check the named series and cultivar, rather than treating every yellow or orange marigold as the same option.
Think about the planting, not the nursery tray
Plants packed closely together for sale can hide their natural outline. Give each variety the spacing recommended for that plant. A tray that looks full on the bench is not a planting plan.
If mixing varieties, compare their expected growth as well as their flowering colour. A taller neighbour can make a small variety difficult to see or reach for deadheading.
Keep the labels until the end of the season. A photograph of the mature planting, paired with the cultivar names, is a useful shopping record for next year. It also helps distinguish a variety that did not suit the design from a plant that struggled because of its position.
Related reading: Marigold Varieties: Discovery, Antigua and Other Series.