I've tried looking through here for the same issue. It seems like this has been an issue for awhile. When I upload my vectors the preview image colors are VERY different and horrible looking. I tried the suggestions of adding my own jpg preview image to a zipped file along with the vector but apparently adobe no longer accepts zip files. Has anyone found a solution? I use inkscape, for reference.
We do accept .zip folders for AI and EPS vectors. Since you are using Inkscape, are you submitting SVG vectors? If so, then you are correct, you cannot zip SVG files with a jpg. You'll need to convert to AI or EPS then zip with a jpg preview in sRGB color profile to correct this issue.
The zipped file I uploaded included a jpeg file and an eps file. I've tried a few times and get this error message. "thanks-pack.zip couldn't be uploaded because it is not a valid file type."