sorry if its a dumb question but new to the game. after firing with speeder bikes, if spending an aim token, which colour dice are re-rolled

Whichever dice you want to reroll, you can reroll any 2 , so if you roll 2 blanks on red you can use them two white. Use them

As Sryath notes, you can pick any two dice to reroll (or more if you have precise), including dice that are hits. Perhaps you're up against armour and want crits for example.

Note, all rerolls must be done in one go though, so if you roll 4 dice, get 3 crits and one miss and have aim. You can re-roll any two dice, or just the blank die, but cannot reroll the blank die get a miss and reroll it again*. This is much like han's uncanny luck, where you can reroll up to three dice.

*Except, _I think_, if you have multiple aim tokens (anyone clarify that?)

20 minutes ago, DwainDibbly said:

*Except, _I think_, if you have multiple aim tokens (anyone clarify that?)

Correct. Here's the exact rule:

"A unit can reroll the same die multiple times by spending multiple aim tokens; however, each die can only be rerolled once per aim token."