UPDATE 2
If you are using com.google.android.material.floatingactionbutton.FloatingActionButton, use app:tint
app:tint="@android:color/white"
UPDATE
Refer to the answer of @Saleem Khan which is the standard way to set the app:tint using:
android:tint="@android:color/white"
via XML on FloatingActionButton.
OLD (June 2015)
This answer was written before October 2015, when android:tint on FloatingActionButton was supported only with API >= 21.
You can change it programmatically using ColorFilter.
//get the drawable
Drawable myFabSrc = getResources().getDrawable(android.R.drawable.ic_input_add);
//copy it in a new one
Drawable willBeWhite = myFabSrc.getConstantState().newDrawable();
//set the color filter, you can use also Mode.SRC_ATOP
willBeWhite.mutate().setColorFilter(Color.WHITE, PorterDuff.Mode.MULTIPLY);
//set it to your fab button initialized before
myFabName.setImageDrawable(willBeWhite);