My plants don't have any color to the stems.
How Do I Know When to Harvest Rhubarb
Certified GKH Gardening Expert
When the stalks start turning red or magenta in color, it means that the rhubarb is ripe and ready for harvest. In addition, their size should be around 10 inches or so. Here is an article that you may find helpful: https://www.gardeningknowhow.com/edible/vegetables/rhubarb/harvesting-rhubarb.htm