Change TextField font color in Material UI?

In Material UI v5, you can just do this using sx prop:

<TextField sx={{ input: { color: 'red' } }} />

A bit longer approach if you want something more reusable:

const options = {
  shouldForwardProp: (prop) => prop !== 'fontColor',
};
const StyledTextField = styled(
  TextField,
  options,
)(({ fontColor }) => ({
  input: {
    color: fontColor,
  },
}));

<StyledTextField label="Outlined" fontColor="green" />
<StyledTextField label="Outlined" fontColor="purple" />

Live Demo