Model/Adminhtml/Source/ApplePayButton.php
public function toOptionArray() {
        return [
            [
                'value' => self::BUTTON_BLACK,
                'label' => __('Black')
            ],
            [
                'value' => self::BUTTON_WHITE,
                'label' => __('White')
            ],
            [
                'value' => self::BUTTON_WHITE_LINE,
                'label' => __('White with line')
            ],
        ];
    }
public function toOptionArray() {
        return [
            [
                'value' => self::BUTTON_BLACK,
                'label' => __('Black')
            ],
            [
                'value' => self::BUTTON_WHITE,
                'label' => __('White')
            ],
            [
                'value' => self::BUTTON_WHITE_LINE,
                'label' => __('White with line')
            ],
        ];
    }
public function toOptionArray ( ) { return [ [ 'value' => self :: BUTTON_BLACK , 'label' => __ ( 'Black' ) ] , [ 'value' => self :: BUTTON_WHITE , 'label' => __ ( 'White' ) ] , [ 'value' => self :: BUTTON_WHITE_LINE , 'label' => __ ( 'White with line' ) ] , ] ; }
Possible Apple Pay button styles

@return array
Possible Apple Pay button styles