Hi there,
the new shoots often are reddish/ purple, especially of cultivars with purple/red flowers, even pink ones. My cultivars with white or yellow flowers send up plain green shoots.

How warm do you keep them? My batch of potted Dahlias didn`t do much, because the wheather was so cold and I keep them in an unheated room. Now after some days of sunshine, they do grow a bit taller.

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You could check the tuber by tipping the container carefully, check the crown and look for squishy parts on the tubers.
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Or just wait, as long as those shoots don`t look like they are shrinking, it should be fine.
Have fun with your first Dahlia summer,

bye, Lin

ps: let`s hear whether you got hooked and how many cultivars you gonna order for next year...