If you have a five-person pile of luggage and you throw a sheet of any color over it, do you really think it's going to conceal it? At best it might serve as a deterrent to the most casual of thieves, but to anyone with a bit more gumption all you're doing is telegraphing "hey, I'm trying to conceal a big pile of stuff here".
1. If you're planning to use a car for luggage storage I'd really be giving strong consideration to renting a car with a trunk even if it was a bit more expensive or inconvenient than a mini van.
2. More critical than covering your possessions are your choices in parking spaces. You want to park either where there's a lot of foot traffic (reads witnesses) around, or in an attended multistory garage in one of the upper floors away from stairwells or other routes of hurried egress. You definitely don't want to park in alleys or other areas where a thief can work out of sight. Broken window (safety, the stuff that's in little chunks) glass on the ground is a sure indicator that you don't want to be parking anywhere in the vicinity.
3. In addition to a sheet you should also schlep one of those dust-ruffle thingies which you should snap around the bottom of your mini van to conceal the wheels, as well as pillow shams for the thieves to haul their swag away (pillow cases are so cliche). Make sure that none of the three match each other and that at least one is one of last year's floral patterns and another is some strongly colored geometric. Any self-respecting thief will be horrified by the tackiness of the whole ensemble and will move on to a more tastefully outfitted vehicle.