Changing NumberPicker divider color

This worked for me without using the reflection.

my_layout.xml

<NumberPicker
   ...
   android:theme="@style/DefaultNumberPickerTheme" />

Styles.xml (AppTheme is my app theme in the app)

<style name="DefaultNumberPickerTheme" parent="AppTheme">
        <item name="colorControlNormal">@color/dividerColor</item>
</style>

If you want simply to change the color (based on stannums answer):

private void setDividerColor(NumberPicker picker, int color) {

    java.lang.reflect.Field[] pickerFields = NumberPicker.class.getDeclaredFields();
    for (java.lang.reflect.Field pf : pickerFields) {
        if (pf.getName().equals("mSelectionDivider")) {
            pf.setAccessible(true);
            try {
                ColorDrawable colorDrawable = new ColorDrawable(color);
                pf.set(picker, colorDrawable);
            } catch (IllegalArgumentException e) {
                e.printStackTrace();
            } catch (Resources.NotFoundException e) {
                e.printStackTrace();
            }
            catch (IllegalAccessException e) {
                e.printStackTrace();
            }
            break;
        }
    }
}

And after that

setDividerColor(mNumberPicker, Color.GREEN);