No Monthly Fee Debit Card
To custom color balance when light source is camera-mounted, or if exposure is more than 1/30s long, ExpoDisc will not work. I shoot a black/gray/white card, then go into Gimp Levels, and eye-dropper black and gray in Input Levels. Then eye-dropper gray in Output Levels. Save.

Layers, Duplicate Layer.

Dialogs, Layers, Transparency 33-60% as preview shows to be appropriate.

Layers, Merge Down.

That is how I correct color using a black/gray/white card.