Jones and Rosso gave a formula for the colored Jones polynomial of a torus
knot, colored by an irreducible representation of a simple Lie algebra. The
Jones-Rosso formula involves a plethysm function, unknown in general. Our main
result is an explicit formula for the second plethysm of an arbitrary
representation of $\fsl_3$, which allows us to give an explicit formula for the
colored Jones polynomial of the trefoil (and more generally, for $T(2,b)$ torus
knots). Our formula is different from the one given by R. Lawrence, and allows
us to verify the $\fsl_3$-Degree Conjecture for the trefoil, to compute
efficiently the $\fsl_3$ Witten-Reshetikhin-Turaev invariants of the Poincare
sphere, and to guess a Groebner basis for recursion ideal of the $\fsl_3$
colored Jones polynomial of the trefoil.