Your sunlight has lots of UV
The target planet has a tropical climate, also on higher latitudes. Or it has less means to protect the surface from UV.
A white skin will reflect more heat, but it will also be more vulnerable for sun burn. Your colonist children, born underways, will get a metallic looking, dark-blue color skin, with reflective properties and lots of protective pigment.
They could also have been rendered black, purple or brownish, but considering the length of travel, the first generation colonizers decided to use dark blue, for psychological reasons. Because it is exotic.
In a mixed crew population underways, a darker and recognizable skin color in children could pose issues with rejection, or racism. A neutral solution was needed, so the skin color became exotic for all children. In three generations, your population will be dark blue, the Earthly skin color differences have been eliminated.