Although the creators have chosen a lighter (brown) color for Black Piet it is traditional to show Piet with a black skin color because Black Piet gains it's unnatural skin color by traveling through the chimney and thus spreading soot on his or her face.
Chances are Bassie and Adriaan did not choose the alternative color for Black Pete because this program was made by the channel TROS. The weekly drama series of Bassie and Adriaan was made by Bassie and Adriaan themselves, while on an a music album of this duo Black Pete was shown in it's original black color.