Open the BP_AmmoCounter and create a new variable called LowAmmoTextColor and set its type to SlateColor. Then compile and set the Default Valur color to red.

Then create a new variable called LowAmmoPercent then set its type to Float then compile and set its Default Value to 30, this means when the weapon is < 30% ammo the LowAmmoTextColor is used for the current ammo text. You can set this to any value you like.

Then copy the image below in the GetVisibility_0 function.