Answer: Reddening on a golden barberry would indicate stress, yes. It could be due to underwatering, or it might be due to poor rooting. This is a very drought tolerant shrub once established, however it does need an evenly moist soil for about the first year. The goal would be damp like a wrung out sponge, not sopping wet or sarturated and not bone dry. To know if you need to water, dig into the soil with your finger. If it is still damp, do not water yet. If it is drying, water slowly and thoroughly so it soaks down deep to the deepest roots. It is better to water deeply but less often than to water lightly and frequently. After watering, wait a few hours and then dig down and see how far the water went; it can be surprising. Also, use an organic mulch several inches thick in a flat layer over the entire root area (but do not allow it to touch the stems) this will help keep the soil moister.
If you do not think it is a watering issue, it might be due to poor rooting -- this can happen for instance if the soil was not loosened over a wide area at planting time so the roots are not growing outward, if there were encircling roots that were not cut or loosened and directed outward at planting time so they are growing in a circle, or if the soil is poorly drained and thus overly wet and causing rotting.
If you think it is one of these latter causes, you could try digging it up carefully and either transplanting or resetting it. I hope this helps you trouble shoot.
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