Tan leather was first offered in 1999 and became the only color interior offered for T-red and Tropic Green SVTs for 99-2000. Also for those years, black had the option of either tan or blue, and Silver Frost only had blue interior.
However, I don't know if Prairie Tan is the same color interior as the tan offered on pre98 Contours. I know the blues and grays between pre98 and 98+ are different shades. Sometimes it's not a noticeable enough difference to matter (for example, I put a pre98-colored dome light bracket in my Greystone interior) but with seats you may want to verify before ordering.
