Answer: The red coloration is due to pigments called anthocyanins that are formed in the leaf stalks. Anthocyanins form only in cool temperatures and abundant sunlight.
It's quite common for the stalks not to turn red and there's not always an obvious answer as to why this happens. Some experts say lack of light, others say high temperatures while the stalk is growing, yet other factors such as high nitrogen fertilisers could also play a part. There is also a color difference between varieties.
There are a few diseases that can cause problems with rhubarb and they tend to occur most when drainage is poor or when temperature and humidity are high. Yellowing and wilting of the leaves could simply be lack of water at crucial times - the broad leaves need copious amounts of moisture in summer. Or it could be a fungal disease. Check the crown of the plant at the base of the leaves close to soil level for white fungal growth or rot. If you find any, it's best to remove the plant completely and buy a new, healthy one to plant elsewhere in the garden.
Other tips are to feed regularly in the growing season with a high potassium fertiliser and to remove any flowering stems as soon as they appear. It is also worth remembering that rhubarb can tolerate shade, so you don't have to plant it in a sunny spot.
If your rhubarb is growing in any shade or if spring temperatures are warm, the full potential of red color may not develop.
Although this information doesn't solve your problem, it may give you a few potential causes to look into.
Best wishes with your rhubarb!
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