Tier 1 allows for a large range of leeway in Undersuit construction, both color and material. Undersuit accuracy is not much of a factor until Tier 2 and 3, and obviously each version of the undersuit is individually compared to the in game reference.
The format for the the awarded DIN is "S-[Number.]" We understand that in Canon only SPARTAN-IIs had an operating number of S-[Number], but what you are being awarded through this program is your Division Identity Number, meaning the unique identifier that tracks you through this Program and in the 405th itself. While inspired by the Canon operating number system, it is not an attempt to perfectly recreate it, hence why all 405th members, Spartan, ODST, Marine, AI, and Covenant are given a DIN in the same format.
There is no rule or anything that you can only display your awarded DIN on your armor, so if you want you can put B126 on your armor, or any other number that you want, but on your Deployment Certificate and on your Profile it will be listed as S126.