Two colour printing with an X2D

Hi Everyone, newbie to the Bambu Lab ecosystem. I have just purchased an X2D combo and have ran a few single colour prints through it and, so far, the results have been outstanding. I’ve upgraded from ( a much modified) Ender 3 and have been very pleasantly surprised at how easy it is to print with X2D, no more spending what seemed like hours tweaking the settings and the printer, just to get the first layer down! I have just tried to slice my first model using 2 colours in BLS, but am having difficulty getting it two use one colour from the AMS and a second colour from the auxiliary extruder. I have 4 different colours in the AMS and 5th external colour (all Sunlu PLA+). I want to use the first bay in the AMS as the primary colour and the external filament to highlight some lettering.

However, when I slice it in BLS, it wants to use the first 2 colours in the AMS. If I do manage to get it to try to use the first bay in the AMS and the external filament, it always wants to print the primary colour from the external filament (I have grey in the first bay of the AMS and Black on the auxiliary extruder, I want to print the main body in grey and the lettering in black but it wants to print the main body as black with grey lettering, if that makes sense!!) and I cannot find a way to swap them over. If I try to change settings again, the slicer then produces a print that uses only 1 colour from the AMS. All extremely frustrating so any help from all the knowledgeable people on the forum would be greatly appreciated!

Keith