Here’s a chart with what the colours correspond to in dB attenuation and voltage output
So turn it such that the volume buttons are both darker purple.
You don’t need to worry about it being EXACTLY 2V though.
Devices vary a fair bit in output and 2V is not a hard rule. You’re fine to have it be over/under a bit. In fact as long as the amp you’re connecting it to is fine with hotter inputs, you’ll get more dynamic range by having it set slightly higher