The cost for coloured items adds manufacturing cost of at least $0.01. A margin loss too magnificent for Apple to apply.
For example the Macbook Pro's only got Space Black after they resized the heatsink to less than half (50%) the original to use those savings for introducing Space Black.
The iPhone 17 Pro models have one of the most expensive colours (orange) which is 0.02 ct. They only could use that colour by removing or cheap out on proper electro shielding and speaker quality. Hence all iPhone 17 Pro models make an electrical static hissing noise when charging and distortion when speakers are set on max loudness.
The same goes for Airpods Pro 3, a small redesign that requires new molds. So even with this product they cheapen out in proper shielding (hence the static noise during ANC and the same H2 chip).
Same for HomePod, they only re-introduced it after downgrading and removing the Wifi chip and microphones in that product.
The Apple Watch 11 is exactly the same as Series 10. The reason is because the profit on this product category is declining rapidly. Each product needs to meet certain revenue/profit to get assigned a budget for new features. If it does not meet the financial requirements, it will stall.
Thats's why all regular iPhones will not be upgraded each year.
The only exception (although within borders) is iPhone Pro line-up.
This is also the exact same reason why, after news came out that iPhone 17 Pro are one of the best sold iPhones, got news published they will add small design upgrades, because it already met its yearly quota.
Nothing comes for free.