Grey IMS for spells

What would make a sorcerer’s IMS, for damaging spells, gray shade? Gray Will?

Spell damage is based on Will. So Will.

I thought they needed both gray skill and gray Will? Or was that in Revised?

That might’ve been Revised? But in Gold it’s definitely just Will.

It’s always just been Gray Will for damage. Gray Sorcery gets you 3+ on successes.