Translation:The color of this skirt is not yellow.
Yes, to me it also sounds like "Rack" (native speaker). But be aware: different users may get different audio. (Yes, this is very unfortunate)
I was called wrong for "Rock"="coat," since the sex of the wearer is unknown. ''Skirt" was wanted, as if the wearer were female. Was I correct?
The standard translation of "Rock" is "skirt". This is the most common meaning and the one Germans think of if they hear the word without further context. However, "Rock" can indeed mean "coat" or "jacket". So your answer is not wrong. But this usage is somewhat dated in Germany. Normally, if context doesn't tell you otherwise, you should go with "Rock"="skirt". In Switzerland, "Rock" in the sense of "jacket" or "dress" is still quite common, though.
I believe "Rock" is simply "skirt" in English. "Der Mantel" is coat, and "die Jacke" is jacket.