Yes, it is, Dye doesn't affect the shape of the disc.
PDGA 802.01 rule says : "C. Players may not make post-production modification of discs which alter their original flight characteristics. This rule does not forbid inevitable wear and tear from usage during play or the moderate sanding of discs to smooth molding imperfections or scrape marks. Discs excessively sanded or painted with a material of detectable thickness are illegal. See sections 802.01 D, E and F. "