I don't know which exact genes are making the color difference in the birds you see, but obviously there are some differences. I assume the differences in the Cornish came from the same ancestors that caused the change in body shape.
Chicken color genes can be confusing.
Sometimes the same...
Dark Cornish hens have black double lacing on a rich red/mahogany color. Roosters have much more black than the hens, and do not look very laced.
For photos of Dark Cornish, I suggest this page:
https://www.feathersite.com/Poultry/CGA/Cornish/BRKCornish.html
In the US, the Dark Cornish seem to...
Sorry, I can't help with that particular question.
I understand the basic genetics of Dark Brahmas and Dark Cornish, but I don't think either one is the same as "Dark" in Shamo and Malay.