If line 34, modified federal AGI, is negative, TT leaves line 35 blank, requiring manual fill-in. However if line 34 is positive, it computes the refund automatically. You can see this by fiddling numbers upstream, or just overriding line 31.
Colorado online information in page 14 of the form 104 instructions clearly says that a negative AGI does not preclude TABOR refund.
