They've given you two letters and four numbers ? If so, then it has to be "two and four" as the numbers shouldn't be split.
Pre-and early-war number plate brackets were all-over gloss black. As you're fitting one out of necessity to a later bike, it's up to you what you think looks best really, there is no right or wrong.
As Rik said, they would have been black all over, but since they were deleted early on in the war. "Artistic Licence" prevails to do whatever you want. Ron