Article numbers. The one field every other system will join on. Worth deciding once, deliberately.
What the number is for
An article number is not a description. It is the handle that a tech pack, a sample round, a purchase order, a warehouse and an accounting system all use to mean the same garment. Everything else about the article can change — its name, its colourway, its supplier, its price — and the number has to survive all of it.
Meaningful or meaningless
A meaningful number encodes something: season, division, category. It reads well in a list and it goes wrong the first time an article moves between the things it encodes — a style carried over to the next season, a jacket recategorised as outerwear. A meaningless number encodes nothing and never lies. If you want to read structure at a glance, put it in a field that can change, and let the number be a number.
One number, one article
The commonest failure is a number that means different things in different places: a style number in design, a supplier reference in production, a SKU in the warehouse. Decide which one is the article, and make the others attributes of it rather than rivals to it.
Colourways and sizes
A colourway is not a different article, and a size is not either. Both are dimensions of the same one. Numbering them separately doubles the records you maintain and halves the chance that a change reaches all of them.