Correlation of ranks of labels?

Hi,

I have two ranks of labels (strings) representing user preferences of colors.
For instance, here is a simple example with 4 preferences for each user:

rank1

[1] "red"   "blue"  "green"  "black"

rank2

[1] "white" "gray"  "black" "blue"

How can I compute Kendall's Tau for this scenario?

Thanks in advance,
--
S?rgio Nunes