I don't think there is a native way on Echo to control the hue colors (unless I'm unaware of a command). You can say "Set xyz to 50%" in terms of dimming the lights. You can however set colours via IFTTT (have a read up of you don't know what this is). You can set ifttt to set your hue bulbs for different colours & then tell alexa "trigger sunset" or "trigger purple", having created these commands within the IFTTT. (or applied someone else's "recipe" if it does what you want). Don't worry if none of this means anything to you, it's actually very easy one you've done some reading!