There were three traditional colours for the centre block.
Red for the fire in the hearth.
Yellow for the light in the window.
Black representing a recent death.
Today ... any colour in the centre ... or if doing a scrappy, you don't need to be consistent in the centre block.
Logs can be whatever size you want. I saw one done the other day wiht 6" logs ... and have seen them as small as 1" logs (before stitching!)
Personally I like the 1" finished (cut at 1-1/2") or 1-1/2" finished logs ... keeps it a little more delicate. But that's personal!
Anything goes !!!!!!