This is a phrase used by Northern Ireland folk who use diesel in their car as a cheap way for fuel rather pay for diesel and petrol although this has consequences.
This is phrase is mainly used by farmers and rural folk but can be used as an exclamation.
"(I'm) On The Red"

John: Do you use diesel in your car?
Q-Pat: No, I don't use diesel.
John: So you use petrol then?
Q-Pat: No, I don't use petrol either.
John: What fuel do you use then?
Q-Pat: "(I'm) On The Red".
John: You fecking eejit.
by SuckinDieselBae August 11, 2016
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