When a plant is removed from its container for transplanting, the colour of its roots can give an indication of that plant's heath in addition to any cues that come from the foliage.
Healthy young roots should generally be bright white in colour (accounting for variation between species). Whilst there are exceptions to this rule (our Japanese Blood Grass has red and yellow pigmented roots), healthy roots for many species should looks something like those shown in the following image.
Unhealthy roots will not be a white colour. The following image shows an unhealthy plant that has been in its container for far too long. Notice how the roots have discoloured significantly?
Different species have different types of roots that will vary in structure and colour. Some species have fine and fibrous roots (such as azaleas) whilst others will have thicker roots that tend to girdle (run in circles around the pot). Whilst a dark root colour isn't always an indicator of poor plant health, it may indicate a problem with many common garden species.
In the majority of cases, plants will quickly recover once planted into a larger container or into a garden bed even if the roots don't look especially healthy. That said, it's always good to start with the very best.
At Plants by Adam, we take plant health very seriously and always ensure that the very best quality seedlings are offered for sale. Roots may not be visible to the eye in the same way as the foliage, but their health is just as important for the growth of garden plants.