One helpful tool is the Philatelic Foundation database.
here is a search for J3:
http://pfsearch.org/pfsearch/pf_grd...lledfrom=lkpMy personal opinion: color charts are pretty useless for stamps. On US postage dues, experience is the best guide, and if inexperienced - then compare with known examples e.g. the PF site. Many dealers get the US PD's wrong, almost always to their advantage.
ebay can be treacherous, but sites like the APS sales site are no better. If you are going to spend real money on these, try to put together a reference collection of the colors on damaged low values.
A disclaimer about the PF site, computer monitors can sometimes misrepresent colors - but I find the PF very useful online for these, and other confusing US stamps.
If you want an opinion on your stamp - please post a picture.