I have a datepicker that's showing a header on Lollipop, it looks like this

I want to either change the color of the big date in the header from black to white, or remove the header altogether, I don't care which. I've tried changing textColorPrimary, textColor and titleTextColor but it has no effect.

Well I have no clue what's the parent theme that you have for your Date Picker theme. Let's say you have a custom theme for your Date Picker like below,

 <style name="yourCustomStyle" parent="Theme.AppCompat.Light">

Now Ctrl + click on Theme.AppCompat.Light which leads you to a new way ;) where you can find what you are looking for meaning here your issue is only about the head text but you might wish to change another views color, so that's the place you need to have a look.

And as the answer create a custom theme like below and add this attribute with the color you like

android:textColorPrimaryInverse

should do the trick for you.

   <style name="yourCustomStyle" parent="Theme.AppCompat.Light">
        <item name="colorAccent">@color/blue</item>
        <item name="android:textColorPrimaryInverse">@color/yellow</item>
        .. other
    </style>

Feel free to use your own colors and code(I copied code from this) and it will do the job!

new DatePickerDialog(MainActivity.this, R.style.yourCustomStyle, new DatePickerDialog.OnDateSetListener() {
    @Override
    public void onDateSet(DatePicker view, int year, int monthOfYear, int dayOfMonth) {
      
    }
}, 2015, 02, 26).show();

_{Image : android:textColorPrimaryInverse with Theme.AppCompat.Dialog}