Text color not working in Material-UI Theme

In order to have white text for both colors, you want:

const colortheme = createMuiTheme({
  palette: {
    primary: { main: "#e91e63", contrastText: "#fff" },
    secondary: { main: "#03a9f4", contrastText: "#fff" }
  }
});

The contrastText must be specified within each color intention.

Here's a full example showing this:

Documentation: https://material-ui.com/customization/palette/#providing-the-colors-directly