I am using the Cadasio addin for Onshape and all of my colors are incorrect in Cadasio. The hex colors are assigned to the entire part in Onshape but Cadasio shows completely different hex colors. This is causing a lot of rework once the assemblies are in Cadasio.
I believe a member of the support team has been in touch and is communicating with you via email to get investigate this further. It would be best to carry on directly with them but feel free to update this if necessary