With kale crisps you are ideally trying to dehydrate instead of baking. This saves the chlorphyll from degradation. Depending on your green/veg chlorophyll starts to degrade between 100F-185F (40C-85C). Now, 220F-240F (100C-120C) is as low as most domestic ovens go. However, if you are able to use a lower temprature, that would give you a greener result and a cleaner flavour. But it will take longer to crisp as you lower the temprature.
Using a dehydrator would be ideal, running at around 140F (60C). Think of dryed fruit, herbs or even tomatoes, they are almost never baked, but rather dehydrated. You'll also find that commercial kale crisps use a very low temprature and they are promoted by it as it keeps nutrients intact and preserves chlorophyll.
A tip if you have to use your oven, keep checking and take them out before they crisp up completely. As they cool, the residual heat will dehydrate them a touch more.