It could be a phosphorus deficiency...especially if it's mostly under the leaf or you notice the green of the leaf going to a green/blue hue along with the red.

That would be my guess too. Are they cold? Some plants have a difficult time absorbing P if they are young and to cold. It happens to a few of my annuals every year if I plant them out a to early. They recover when they warm up.

The little critters aren't cold. 70F Thought I might add a photo for clarification

Here is a link that might be useful: Lively Dirt - The Garden Blog

Those look like beets to me.

No - the leaves are clearly rough and lobed at the bottom of the leaf blade, unlike beets. They're just discoloured radishes.

Have they had any fertilizer? I ask because the pale color often says "We need nitrogen."

Looks like low P to me too, notice it's on the older leaves?