Some years ago I had a slide with two blue bars, one on top and the other one at the bottom.

“The upper one is darker”

“They have the some colour code” was my answer. The wrong one.

Indeed they were the same colour, but they looked different. I erred in answering, and I fixed the colour.

It’s not about maths or physics, it is about what your audience sees. This is true in many instances including the vertical alignment of elements that normally doesn’t match the one that our eyes perceive (the horizontal is generally not a problem on the contrary).

Introduction and disclaimer are here´╗┐