Since SP doesn't come into play when you're using Soul Cards, it doesn't matter what color they are. However, there are two things to consider. First, many soul cards work off your Sphere Levels. Archer Scout is a Gowen card and her soul skill does 20 damage per point of Gowen Sphere level. Therefore, if you're planning to not use any Gowen cards and have 0 points in Gowen's Sphere Level, that is not the soul card for you. From the opposite point of view, you can only have 3 copies of any one card in your file, soul cards included. That means that if you want 3 Zagars ready for action, you can't use him as a soul card also. In this way, it's better to use other colors for your soul cards.
"Scissors are overpowered. Rock is fine." -Paper