Your silkie x Sumatra isn’t black breasted red. You mentioned that the silkie involved in the cross has white. If that extra white is on its hackles, saddle feathers, or chest, but not in any sort of pattern, then it’s called silver leakage. It’s very common for silkies to have leakage, because they are often crossed between varieties. And, because of their silkied feathering, it’s not as important (to hatcheries and backyard keepers, at least) to breed the leakage out.
Here’s how it happened- your silkie passed on their leakage to their son. However, the Sumatra must have had a gold gene (all solid black birds have a gold and/or silver gene, even if they don’t have leakage). So, instead of silver leakage, the mixed roo got gold/red leakage from the gold gene he inherited from the Sumatra.
You see, the leakage/pattern is from the silkie, but the color (gold/red) is from the Sumatra.
I will respond to this on
@NatJ ’s thread.