In our official docs we have css and scss variables for most of the components that can be customized. In your example you should change those two and declare them before importing mdb.scss file.

$tooltip-color: red;
$tooltip-bg: cyan;
@import "mdb-angular-ui-kit/assets/scss/mdb.scss";

In case of theming you need to overwrite the styles manually in specific classes. That is because theming resolves after initialization of scss variables. Here is example:

@import "mdb-angular-ui-kit/assets/scss/mdb.scss";

$my-theme-primary: #9c27b0; // theme primary color, change this value to customize theme
$my-theme-secondary: #69f0ae; // theme secondary color, change this value to customize theme

$my-theme: mdb-light-theme(
  $my-theme-primary,
  $my-theme-secondary
); // create the new theme using primary and secondary colors

// include theme styles
@include mdb-theme($my-theme);

.tooltip-inner {
  color: red;
  background-color: cyan;
}