TWO SHADES?

[ATK]

So, say that I have a blazer and pants. The blazer is red and the pants are a smidge lighter. Would that work?

 

[RabidOrphan]

If you wear two shades of the same color, people will mistake it for an old suit; suit's pants usually get faded faster than the jacket. A pair of black or white pants will work with your red blazer.

 

[Anachronist]

+1
Two similar shades of the same color is difficult. You're on the safer side playing the contrast game and typically create much more striking looks (e.g. if you want to go color: red chinos with navy blazer or in your example navy or white pants with the red jacket).

 

[maxalex]

The main problem with a slightly different shade of red trouser is that you will look like you’re trying too hard—and failing. In some ways your failure will be better than success because a matching red suit will make you look like a bar mitzvah DJ or a Pitti peacock. So this is a case where you want neither failure nor success but something completely different.

Such as? A contrasting trouser, perhaps in charcoal or khaki. That way you will look like you just threw a couple of things together (which happen to click stylishly), and no one will ask you to play the limbo song.