What happens if i used capes mod in the speedrun,what would happen to my run?

pretty sure that'll get the run rejected, modifying the cape files on your computer is allowed though and there's a really good tutorial by Secondairy on the topic

Using any mods that are not in the allowed mods list will get your run rejected. You can see all of the allowed mods here https://redlime.github.io/MCSRMods/

Ok,thx but what's the message for rejecting the speedrun(AKA the reject reason for using cape mod)

REJECTED: Tails is orange.