Sienna Pearl Coat was available on the 1999. To find your color code, open up your driver-side door and locate the VIN tag on the bottom left corner of the door (if you were looking at the door from sitting in the seat). there will be a place on there that says "paint code" i think...should be just under the bar code.
Once you pull that, here are the color codes for all the available maroonish reds for 1999 -
Chili Pepper Red Pearl - EA or VEA
Sienna Pearl Clearcoat - UL or WUL
once you get what color you need, you can get in contact with
www.paintscratch.com. They'll have what you need. They'll color match perfect.
You can also get touch-up paint from the stealership, but since 2002 was the last year they offered that color on the Grands, they may or may not have touch-up paint for you.