Blue, black, orange, and red lines trace the ancestry of real versions of an element. You can use the real version ancestry to determine the closest common ancestor of any two versions. The closest common ancestor is the most recent version upon which the two versions are both based, by some combination of direct ancestor and merge connections. (When considering a virtual version in a closest-common-ancestor analysis, first follow the green line back to the corresponding real version.)
You can also use the AccuRev Command Line Interface command accurev anc -c to find the closest common ancestor of two versions.
In the Merge command, AccuRev determines the closest common ancestor of the two versions to be merged, and uses this version to perform a three-way merge. AccuRev tracks patch ancestry separately from merge ancestry. In determining the closest common ancestor of two versions for a merge operation, AccuRev takes into account previous merge operations, but not previous patch or revert operations.