In this project, a sprite with a costume that was exported from Inkscape appears gray:
http://scratch.mit.edu/projects/25427615/

There is no issue with the costume, as it has the correct colors when downloaded as an SVG file and opened in Inkscape or web browser.

This appears to occur when the graphics file has any “linearGradient” elements that have only one “stop” element. Instead of interpreting them as a solid color, Scratch colors these shapes gray.