It clicked on a day I was supposed to be travelling light. Laptop, a change of shoes, a wet umbrella, a sandwich that leaked through its bag and somehow still ended up fine because everything had its own compartment and nothing touched. I'd spent years carrying a tote with everything loose at the bottom like a jumble sale, and that was the first time I got off a train and thought, right, I'm never going back.
On colour, I've made my peace with the fact that pale ones look wonderful for about three weeks. A light tan or a stone colour photographs beautifully and then meets a bus seat and that's that. Black is the obvious answer and it is boring, I won't pretend otherwise, but it goes with everything and hides the ink stain from the pen that always seems to explode in there. A proper dark brown does the same job with a bit more warmth to it. If you want some personality without the maintenance, a deep green or a burgundy in a slightly textured fabric hides scuffs far better than anything smooth and pale ever will.
If you've got none of these and you're starting from nothing, go for a mid sized one in a structured fabric, something that holds its shape when it's half empty rather than slumping into a sad little sack, with a laptop sleeve even if you don't currently own a laptop, because you will end up carrying someone else's at some point. The mistake is buying the huge one because it looks like good value for the extra space. You will fill it, every time, with things you didn't need to bring, and your back will know about it by the second stop on the train.