Unless you monitor the temperature, you won't be able to really get that pink color by slow cooking. Reason being, the meat will usually have to cook longer in order to get tender. Dry roasting will be the easiest for you to get the interior color, but it won't leave the meat very tender. Sirloin cuts tend to be very low in fat so you either have to quickly cook them to medium rare max, or slow cook them until the connective tissues break down. Searing will help give you color and help to give the braising liquid flavor if you plan on using that as a sauce. Braising in an oven or using a crockpot will yield similar results so you can use either one.
"Boldness has genius, power, and magic in it."
Johann Wolfgang Von Goethe