What version of Tableau are you using?
Can I suggest you check the known issues list (Known Issues | Tableau Software ) and the release notes lists Tableau Desktop | Tableau Software . I've seen several fixes that relate to issues with the colour palette and shapes. It may be you're experiencing a known issue that has been fixed in a later version of Tableau than you currently have.
eg in 10.3.2 this issue was resolved
and this issue resolved in 10.2.1
If you can't find an issue listed, then I suggest you raise it to Tableau Support.