You might have both black trousers and black pants, but you surely have non-black trousers and non-black pants as well. Taking it as a mathematic problem, the probability of wearing black trousers and black pants at the same time shouldn’t be very high.
However, if we do it deliberately, we can easily make it. For example, we like black things, we have almost all of our trousers and pants in black. Or we could have a strangely formed habit of only wearing trousers and pants in identical colour.
However, most of the time, most of us wouldn’t spend time on matching trousers and pants in any way. Not so many people will see the colour of our pants, why bother?
All I am trying to say is that although we may have possessed black trousers and black pants for years, we may have never worn them at the same time. Even if we did, we might not aware of the very fact and the effect it brings——
When we wear black trousers and black pants at the same time, we seem not wearing pants at all.