With the release of Legacy TE v2.5 comes DukeItOut's Unbounded Team Color Engine. While incompatible with Ex-based builds in its original state, DesiacX has made a branching version for Ex-based builds. This guide will tell you how to implement this system, based on the type of build you use.
Additionally, I strongly recommend implementing DukeItOut's 50CC into your build before starting. I have a separate guide on how to implement 50CC into your build, which you can follow with the link below.
http://forums.kc-mm.com/index.php?topic=80226.0So with all that out of the way, let's get started!
First, you need to go through your codeset/GCT and remove the following codes, if present and based on your build type:
Non Ex-Based Builds (builds which don't use the BrawlEx Clone Engine):
Alt Color Engine 1.2 [Dantarion]
Alt Color Engine Data [Dantarion]
Masquerade Costume Flags [ds22]
Three Team Costumes for Ganon Costume ID Fix [PyotrLuzhin]
Three Team Costume Preservation
Ex-Based Builds (builds which use the BrawlEx Clone Engine):
Alt Color Engine 1.2 [Dantarion]
Alt Color Engine Data PMEXv0.5 [Dantarion, PyotrLuzhin]
Masquerade Costume Flags [ds22]
Three Team Costumes for Ganon Costume ID Fix [PyotrLuzhin]
Three Team Costume Preservation
Once the codes above have been removed, insert the following codes into your codeset/GCT, based on your build type:
Non Ex-Based Builds:
Ex-Based Builds:
If you're a non-Ex user, then refer to the video guide below (by MetalLegacy34). While this guide is centered around LTE, this method can still be used for non LTE-based builds. Also take note you will also need to make use of the new .masq format:
https://www.youtube.com/watch?v=heDXDdiUcicHowever, if you're an Ex user, then you'll instead need to modify the CSSSlotConfig file of your Fighter of choice to configure your team costumes. Refer to the image guide (made by ALM5252) below when setting things up:

The following color codes are used for teams:
00: Red Team
01: Blue Team
03: Green Team
And with that, you're done. However, I have one last note to make: When adding costume slots to anyone (regardless of build), avoid using 12 (0xC) and 13 (0xD), as those are the FitFighterDark and FitFighterFake slots respectively. So if you're already using either of these slots, change them after following this guide.