It seems since Android 4.3+ it's no longer possible to make String-resources as follows (not sure if it was ever meant to work this way though):
<string name="my_hint"><font size="13" fgcolor="#ffbbbbbb">Hello world</font></string>
If you do, they turn out white. So you are stuck with creating the String-resource this way:
<string name="my_hint">Hello world</string>
And then use the properties on the TextView/EditText to edit the color of the hint. To change the size, it seems it's still possible to do:
<string name="my_hint"><small><small>Hello world</small></small></string>
I found this out by reading the comment of this answer: https://stackoverflow.com/a/11577658/2422321, by Edward Falk