Intuitive Design using Colour

My mam and I both have Hetty and Henry vaccuum cleaners. Hetty has been part of my life for three or so years and when mam needed to buy a new hoover, I had no hesitation in recommending the Numac family to her.

I noticed that everytime my mam went to turn off her Henry, that she hit the "hi power" button instead of the on/off switch.

This puzzled me. I never got this wrong. We investigated...

On my cleaner, the on/off switch is green and the hi-power switch is red - this must be the intuitive colour for operation.

Which one would you go for to switch it on/off?
Another example... I remember getting a new bank card and noticing that almost everytime I used it, I inserted it at the wrong end... whereas, this had never happened with my previous card.

Unfortunately I can't remember exactly which end I inserted first, but I think it was blue!

Back to first principles

I call this method with which I approach life "back to first principles" without being entirely certain that that term is correct. I remember learning about first principles in maths class in secondary school and a lightbulb flickering on inside my head.
Here's a story to illustrate my meaning...

A new washing machine is delivered and needs to be installed. The old machine had both a hot and cold water input whereas the new machine only has one water input.

The installer rummages through the bits and pieces of plumbing attachments... he can't find what he's looking for. He wants to close off one of the inputs, to cap it. Something like this:

But WHY?! Why add to the detritus? This is the first step in a scenario that could develop into this...

So I suggested "going back to first principles". Remove the excessive y-junction and replace with a more simple piece of plumbing - a straight joint. Simpler than the cap with all the potential problems it can bring.

The installer was full of praise. Dumbstruck in the simplicity of the solution. Surprised by the "out of the box" thinking. And it seemed so obvious to me. More than obvious, it seemed the better solution.