How to change Material UI input underline colour?

You can change the underline color of Select Component using two options

1. Overriding with classes

Create a <Input /> element using input Props and override using classes using underline key.

<Select
            value={this.state.age}
            onChange={this.handleChange}
            input={<Input classes={{
              underline: classes.underline,
            }}
             name="age" id="age-helper" />}>

I applied this in your sandbox and take a look at this here

2. Using MuiThemeProvider

const theme = createMuiTheme({
  palette: {
    primary: green,
  },
});

And apply the theme using <MuiThemeProvider/>

I have applied both in this sandbox

Customising Select