It is called "purpleing" and is primarily a nutrient uptake problem due to their young age and immature or stressed roots.

At this stage of growth the plants have not yet developed the fibrous feeder roots they need. They only have what are called "water roots" (tap roots) so they are totally cotyledon-dependent for nutrients. If they are kept too moist those roots are stressed and the plant begins to develop what appears to be a phosphorous deficiency - thus the discoloring.

In my tests of them I have found that root riot plugs tend to stay too wet. It is difficult to maintain the proper moisture level with them.

To eliminate that problem pot them up into potting mix. I trim off as much of the cube as possible without doing any root damage. Use scissors. Then pot them up. That will trigger fibrous root development.

Once they are potted up and settled - give them a day or so - then a well-diluted to 1/4 strength liquid fertilizer feeding. Keep in mind that peppers prefer to dry out between waterings. They do not like to be constantly moist.

Hope this helps.

Dave

Thank you so much that's very helpful. The route right cubes produce germination so fast, but I have noticed that they stay very wet.

Thank you
Zach

I would not worry about it. I have experienced that often with pepper plants. They have always turned to a nice dark green before long, and always go on to do well, barring plant murder, accident, or natural disaster.