Bennu, like Fenix, and like DIV game studio (father and grandfather of Bennu) never allow to user to select the transparent colour: in 8 bits mode, the first color of the palette, color 0, is the transparent colour, BUT you can set the RGB asociated to it for working with graphics editors.
In 16 bits mode, color 0 is the only transparent color, as same as 8bits, but you cannot set his RGB because EVERY color in 16 bits has an associated RGB value (defined by his bits: 5,6,5 for R,G,B component).
32 bits mode is new in Bennu, an add an alpha channel: its an 8bits extra information for alpha or transparency (8,8,8,8 for R,G,B,A components).
But be careful with collision function, it takes only color 0 as REAL transparent (pixels that not collide with others graphics).