To complicate matters, your 17yr old paint has most certainly changed a little over time from when it was new.
You need paint swatches if you haven't already. What I did was get 20 paint swatches from Prismatic Powders for free. Compare the paint swatches to your paint in the sun, shade and different angles. Take your time and you should be able to get very close.
One other variable for mine since it is Bright Silver Metallic was the top coat which is available in different glossy or satin types of finish and can change to overall look of the base color.
If your paint isn't metallic then it should be a little easier since the metallic flakes come in different sizes and quantity.
I hope you find the match and then hang onto that paint swatch!