Absolutely not. Green rhubarb will have the same flavor as red rhubarb. The color does not indicate "ripeness" when it comes to rhubarb as some are inclined to believe. You can proceed with the recipe as is without making any changes if you use green rhubarb. The syrup may not have the dark pink / red color though. However, if you do want the rhubarb syrup to have a similar color, you can add a small piece of beetroot into the rhubarb juice and sugar when cooking the syrup. The beetroot should gently leech out a pale pink color to the syrup. Then take it out when you have the color you desire because you don't want the syrup to be infused with beetroot flavor.