// Add color for stock status single and variable
add_action( 'woocommerce_after_shop_loop_item_title', 'woowoo_category_stock', 20 );
//add_action( 'woocommerce_after_shop_loop_item', 'woowoo_category_stock', 20 );

function woowoo_category_stock() {
    global $product;
    if ( $product->get_type() == 'simple' ) {
		$stockquantity = $product->get_stock_quantity();
        if ( $stockquantity > 5) {
            echo '<div class="woowoo_green"><i class="fa fa-square" aria-hidden="true" title="auf Lager"></i> - auf Lager</div>';
        }
        elseif ( $stockquantity  >= 1 ) {
            echo '<div class="woowoo_yellow"><i class="fa fa-square" aria-hidden="true" title="wenig auf Lager"></i> - nur wenige verfügbar</div>';
        }
        else {
            echo '<div class="woowoo_red"><i class="fa fa-square" aria-hidden="true" title="nicht verfügbar"></i> - nicht verfügbar</div>';
        }
    } else  if ( $product->get_type() == 'variable' ) {
        foreach ( $product->get_available_variations() as $key ) {
            foreach ( $key['attributes'] as $attr_name => $attr_value ) {
                 if ( $key['max_qty'] > 5 ) {
                    echo '<div class="woowoo_green"><i class="fa fa-square" aria-hidden="true" title="auf Lager"></i> ' . $attr_value . ' - jetzt verfügbar</div>';
                }
                elseif ( $key['max_qty'] >= 1 ) {
                    echo '<div class="woowoo_yellow"><i class="fa fa-square" aria-hidden="true" title="auf Lager"></i> ' . $attr_value . ' - wenige verfügbar</div>';
                }
                else {
                    echo '<div class="woowoo_red"><i class="fa fa-square" aria-hidden="true" title="nicht verfügbar"></i> ' . $attr_value . ' - nicht verfügbar</div>';
                }
            }

		}
    }
}

####   CSS for Shop view and product oage with font awesome icon ####
.woowoo_green { color:#1db214; display:block; text-align:center; font-size:0.85em;clear:both;}
.woowoo_green:before {font-family: FontAwesome; color:#1db214; content:"\f0c8\0020";}
.woowoo_yellow { color:#B29815;display:block;text-align:center;font-size:0.85em;clear:both; }
.woowoo_yellow:before {font-family: FontAwesome; color:#B29815; content:"\f0c8\0020";}
.woowoo_red { color:#B2141D;display:block;text-align:center;font-size:0.85em;clear:both; }
.woowoo_red:before {font-family: FontAwesome; color:#B2141D; content:"\f0c8\0020";}

Shop View -> https://i.imgur.com/3pzSPrl.png
Product View -> https://i.imgur.com/rCicbrV.png