Was watching Tumble Leaf with the kid the other day, and they talked about how looking through a red glass blocks out the red that exists behind it. Which in turn triggered a nostalgia run thinking about board games like Password and Clue Jr. that had cards with light cyan text and red squiggly lines overlaying them that could only (easily) be read when put under a red magnifying glass.
Hadn't really thought about what was going on there before, but here's me trying to figure it all out by using HSL, repeating background gradients, and a multiply mix blend mode. Drag your pointer across the screen to reveal the message.
Seems the common name for this technique is a "red reveal". Boils down to a light cyan "printed" first with a red "noise" layer above (in my case two repeating background gradients at opposing angles). The revealer glass (the topmost layer) is the same color red and has a multiply blend to get the cyan text darker as an enhancement.