Stereochemistry and stereoisomers

Meso structures

Morrison and Boyd § 4.18

Like in 2,3-dichloropentane, four different structures can be made of 2,3-dichlorobutane:

A B C D

Again, arrange the structures in such a way that they form left and right mirror images. Then try to rotate one structure to make it identical ('superimposable') with the others. Which are superimposable, which are mirror images, which are diastereoisomers?
Assign R/S labels to the chiral centers.

Questions:
What is the correct label for the yellow carbon? And the brown one?
yellow:
R
S
brown:
R
S

Continue with Conclusion, and more questions.