The white bag, when exposed to any sunlight coming through the window, will still allow light and heat to enter. If you're going to cover them, use black bags.
Even the black bags will absorb and hold heat which could cause that spindly yellow growth.A white bag laid over the black will allow the sunlight to reflect back.

Comforters, tarps, cardboard - all are better choices.

Anything plastic will promote rot and fungal problems.

Those fabric tarps are a little hard to find (I usually see tarps in plastic). A comforter seems like it would be too heavy. How about a sheet?

If the roses were mine I would use paper grocery sacks, and allow for some venting for air circulation. If you need to keep the light out, why not cover the windows in the garage?

Yes, why not cover windows in the garage Redsox?
Better than wrapping with chances of fungal problems.

Hi. Lets see....grow lights in the windows (I grow orchids also) and covered up windows. What will my neighbors think I am growing?

:-)

I personally would not care what my neighbors thought about my garage windows being covered. lol.

I would care more about having to make my way through the garage in the dark.

Understand that 'dark' means dark. It does not mean shaded, or dim, but dark. Also that the covering does not need to be form fitting. Large appliance boxes work very well.

That's why man invented flashlights & inside garage lighting. ;-)
If your not up for adventure, then large boxes would work well as mad -gallica suggested.
Those can be gotten at your local big box or large appliance stores.

I will try one or the other. Thanks!

Don't be concerned about the effect of artificial light on your garage kept roses. The light intensity of a lightbulb or flashlight isn't powerful enough to initiate growth or provide heat to interfere with a dormant plant. Even the grow lights, which supposedly have a light spectrum similar to sunlight, have to be positioned inches above the plants to be effective. The occasional lighting of the garage lights or use of a flashlight will not be a problem in your garage.
Sunlight coming through a window or even through an uninsulated fiberglass garage door will cause early growth on plants, especially as the sun rises higher in the sky and the length of daylight increases as winter progressses.
There's a tremendous amout of heat in sunlight coming through a window. On a below zero F degree day the space between a storm door and inside door can get quite warm due to sunlight shining through the storm window.