Beetroot and wheatgrass cupcake

I would not use wheatgrass. It has a very specific flavor that doesn’t belong in cupcakes.
The beet powder should be totally fine, maybe swap out 1/4c of flour from the original recipe and use the 1/4c beet powder as a starting point. The beet flavor i think would be better with a chocolate cupcake

I'm not 100% sure if combining wheatgrass and beetroot into one cake would be the way to go, but I think you could easily make a chocolate beetroot cake and then add wheatgrass to the icing or possibly even adding the wheatgrass in and letting it be just a very earthy/chocolatey cake!

https://food52.com/recipes/19271-nigel-slater-s-extremely-moist-chocolate-beet-cake
(maybe based on this?)

I was thinking color wise if you didn't want to do chocolate maybe basing your recipe on this parsley cake (full disclosure, I'm not sure I've ever had wheatgrass powder so I'm not clear on how strong the taste is). You could ice the cupcakes with a simple/healthy glaze(maybe coconut butter based) or just an icing and the beetroot powder for some color and flavor. Possibly?

(The parsley cake recipe I was thinking you could base your cake on)
https://food52.com/recipes/27120-roberta-s-parsley-cake

This sounds like an intriguing challenge! Good luck! Definitely post how it goes-- I would be curious!