There's actually no simple way to do this. The color is coming from a function that determines which color to make the text based on the background color:
For now you can add this to the bottom of your app.core.scss
file:
// Change white to your color
.toolbar[primary] .toolbar-title, .toolbar[primary] .bar-button-default {
color: white;
}
Could you create an issue for it please? We're going to need to discuss this. http://ionicframework.com/submit-issue/