How to let the family know when electricity is cheap or expensive

I’ve recently switched energy tariffs so that I get 3 periods of cheap electricity, 3 periods of standard rate electricity and one period of expensive electricity.

My smart home looks completely analog and the “smart” is hidden in subtle automations. I don’t have a wall tablet, people don’t control it through their phones.

I’m looking for a subtle way (no TTS or mobile phone notifications) to let the family know when it would be a good, ok, or bad time (or open to ideas of more) for putting on boring, high energy jobs such as the drier or dishwasher.

I have so far considered a post-it note on the fridge, disabling the devices with switches and changing one bulb to a light changing one. Non of these (bar the post-it note) have gained any traction.