The short answer is: Probably. However, when it comes to food holding temperatures that word never applies well, so always err on the side of safety. Discard and start fresh, as potatoes prepare quickly and are very economical. As for the coloration, some variation is to be expected, especially if left out long as you describe. The cells will tend to break down as the heat within the potatoes will continue to ‘cook’ the potatoes for a short time and slight color variation is typical in most cases.