I have a blue. Pretty sure the grey ramp passes guarantee a spot because they cost an extra $145.
More info:
"Daily, there are 230 student spaces available in the UND Parking Ramp for "S" permit holders. It is open to anyone for "Pay-as-you-go Parking" 24 hours a day. Overnight parking between the hours of midnight and 5:00 am is prohibited for A, S or H permits and General parking spaces.
Reserved Ramp Permits (RCA) for staff or faculty are also valid in surface "A" (red) and "A/S/H" (brown) lots. Reserved Ramp Permits (RCS) for students are also valid in surface "S" (blue), "A/S/H" (brown), and "H" (green) lots."