The only way to change how a shield looks is to change its mesh.
No textures.
However, you can change the color that the shield has by changing these modifiers in the PlayerRACE's .entity:
shieldData
shieldAbsorbGrowthPerDamage 0.001000
shieldAbsorbDecayRate 0.012500
shieldAbsorbBaseMin 0.150000
shieldColor ffffc705 <----- this one changes the color for all ship's shields for that race.