Okay so, I made bread a few times while february and RED were on my mind. I put in 2 T dried powdered beets per lb loaf of bread to make it noticeably red. It does not taste like beets. We used it for sweet and savory sandwiches and toast and pizza dough, all successful.
We used beet powder (and continue to use it) in smoothies, with apples, mandarins, dried light lettuce powder, matcha powder, fresh broccoli, water. Sometimes I add in 3-4 T of fruit puree no sugar to give it a little more sweetness. Lately we've added peach puree.
The powdered beets seem really lacking any taste. So using it in cookies I doubt they would taste like beets at all. On the positive side, there are nutrients that are positive that are worthwhile adding to our meals.
Then, recently I ground up the dried purple sweet potatoes. The powder is not purple, it is a pink/red (not as dark as beets). When it gets wet it goes to purple, a dark purple. I have plans to try it in bread/pancakes/muffins. Purple sweet potatoes (bought from azure standard) were good in desserts, as baked potatoes, and in frozen desserts. The color was the most attractive quality of the sweet potato. They are not strongly flavored so I wouldn't expect them to cause any issues if used in cookie (like sugar cookies).
If someone feels like starting a GREEN thread in honor of St. Patrick's day, in less than 2 weeks, go for it. I've found that the fresh baby spinach is the greenest of greens and very light on flavor.