See simulation/data/player_defaults.json which has a Color property for each player. Note that this file will be moved and renamed as part of #3355.
Individual maps might still override those settings. The colors (or rather that file) could also be used as part of checksumming mods for multiplayer compatibility so there is a small chance that such a change might cause an OOS in the future (unlikely but still worth noting). Also note that a change to this file will only affect the player color for you, and any maps you produce that don't save the color will have the original default colors for all others.