How can I match an existing stain color on exterior shutters? The bottom half has been faded by sprinkler hitting them
Matching stains on wood is a tricky business.  A paint store should have samples of stained wood you can take home to compare, but differences in wood species can throw off the color of a stain.  It's not likely that the store will be able to "color-match" the stain even if you brought them a shutter - solid colors are easy to match, but transparent ones are nearly impossible, due to the color variations found in the grain of the wood.  If you do decide to try that route, most stores will need a sample of the color to be matched that's about the size of a quarter or slightly larger - which you'll have to extract from your existing shutters.

Another issue you'll face is that stains and wood finishes tend to darken over time as they age and are exposed to UV light.  So, in the off-chance you do find a stain that matches, it may be significantly darker than the existing wood in a few years.

To be perfectly honest, your best bet would be to paint the shutters a solid color, or sand them down and re-stain them with all-new stain.  A solid color will be easy to retouch as the finish is exposed to the elements.

Source: I worked for a big-box retailer for about 5 years, mostly in Flooring, but I also worked in Paint when we were short-handed.  Between the two, I've probably answered most of the common questions about repairing pre-finished wood.