I\'m trying to establish my own palette colors to match my branding in Material UI. So far I can only get the primary and secondary colors to work when applied as the background
Other than needing to change purple in your MyTheme to be something like purple[500], I'm not sure why this wouldn't work for you. Are you sure you can override anything other than the primary and secondary in this way?
Regardless, here's a workaround:
In MyTheme.js:
accent: { backgroundColor: purple[500], color: '#000' }
Then in App.js:
Working example here.