How to change the underline color of selected tab in angular-material?

You need to define a custome theme and set the accent-color to the one that you want your md-ink-bar to have. In this example it's white:

var customAccent = {
    '50': '#b3b3b3',
    '100': '#bfbfbf',
    '200': '#cccccc',
    '300': '#d9d9d9',
    '400': '#e6e6e6',
    '500': '#f2f2f2',
    '600': '#ffffff',
    '700': '#ffffff',
    '800': '#ffffff',
    '900': '#ffffff',
    'A100': '#ffffff',
    'A200': '#fff',
    'A400': '#f2f2f2',
    'A700': '#ffffff'
};
$mdThemingProvider
.definePalette('whiteAccent', customAccent);

$mdThemingProvider.theme('whiteAccentTheme')
    .primaryPalette('deep-purple')
    .accentPalette('whiteAccent');

You can generate an accent-palette on this site: https://angular-md-color.com/#/

In your view, use the new custom theme for your md-tabs:

<div md-theme="whiteAccentTheme">
  <md-tabs class="md-primary">...</md-tabs>
</div>

The simplest way is to change via CSS :

md-tabs.md-default-theme md-ink-bar, md-tabs md-ink-bar {
    color: red !important;
    background-color:red !important;
}

But you can also check the § about theming in the documentation : https://material.angularjs.org/latest/Theming/01_introduction

Sometime the CSS is embedded and generated on the fly in style-tags, if this code doesn't work, try to force the color with !important.

md-tabs md-ink-bar {
    color: green;
    background-color: green;
}