In the following example, @drawable/ic_photo_camera_black_24dp icon is black color but we use app:tint to change it to white.
<android.support.design.widget.FloatingActionButton
android:id="@+id/fab"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="bottom|end"
android:layout_margin="@dimen/fab_margin"
app:tint="@android:color/white"
app:srcCompat="@drawable/ic_photo_camera_black_24dp" />