Reds to be specific.
Right now Im running mix of arm pen and AS.
I play mid Trist quite aggressively, so how would flat AD reds work out?

armor pen. There's no reason to run AD runes on her given that she has no abilities that scale to it. Armor penetration runes are always better when your character simply doesn't have AD ratios. It only gets complicated as those ratios start creeping up, but Tristanna's part of the old crowd where no one batted an eye at AD carries sporting exclusively AP ratios.

I see...but the thing is the AD runes are to boost ur early game and most of Trist's early damage comes from her auto attacks anyways (after level 1 anyways). Whats more as I recall AD helps better with last hitting than arm pen dusnt it?

The hard math is that unless you know for a fact that your opponent is crawling in armor (maybe they have a graves + taric combo, with Rammus in the jungle or something) armor pen starts strong and only gets stronger.

Stacking AD runes to last hit is just lazy. If you could kill them at ~55 HP being able to kill them at ~60 HP confers no real advantage.

Again, unless your character has abilities with AD ratios, AD runes shouldn't even be on the table.

It's easier last hitting in early game, yeah, but ArPen is better vs players at all stages of the game. At least that's what I thought, someone else said otherwise, and I can't be bothered to look up the math now.

It is, however, actually a significant amount of AD for last hitting purposes.

I go with magic pen reds, mana regen yellow and ability blues and quints