Rhubarb produces its tasty pink stems throughout thr spring and early summer. However, you can extend the season by 'forcing' a significantly earlier crop by covering over the crown before it starts to sprout in say January or February with a bucket or an ornamental terracotta rhubarb forcer.
You then need to cover over the bucked with straw to keep the temperature up especially if the weather is cold.
Plants need light to photosynthesise and produce chlorophyll, which in turn makes foliage green. Exclude every last shard of light and plants cannot photosynthesise. Your light-excluded plant will then desperately reach out in search of light, producing smooth, pale stems in the process.
If you want an even earlier crop then you can grow some rhubarb in a greenhouse and force in there, the extra heat and protection as well as the rhubarb forcing pot will bring in on far earlier than in the open allotment.