Here's what described in the documentation, by default it uses the color set in styles.xml attribute colorAccent.
The background color of this aspect defaults to your theme's colorAccent. If you want to modify this at runtime then you can do so via setBackgroundTintList(ColorStateList).
If you want to modify the color:
in XML with attribute app:backgroundTint
<android.support.design.widget.FloatingActionButton
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:src="@drawable/ic_add"
app:backgroundTint="@color/orange"
app:borderWidth="0dp"
app:elevation="6dp"
app:fabSize="normal" >
if you want to modify the icon color, you can try
android:tint="@color/colorname"