Niddah 2:7

Red (in the previous mishna) means like blood from a wound. Black is like the sediment of ink; darker is ritually unclean, while lighter is clean. The color of crocus leaves means the brightest part of the flower. The color of water mixed with earth means like water poured onto dirt from the Beis Kerem valley until the water floats on it. The color of diluted wine means two parts water to one part Sharon wine.

Niddah 3:1

If a woman miscarries a piece of flesh with blood, she is ritually unclean; without blood, she is ritually clean. Rabbi Yehuda says that she’s unclean in either case.