At the Newport boat show, I asked an Awlgrip rep about color-matching. He told me that an Awlgrip technician can visit a boat, with a camera, to take photos and then to digitally match the colors. I would imagine that, with their reputation on the line, they would be able to apply specialized gear and serious technology and expertise in order to make recommendations. Of course the need for that is probably typically much less among "us" than with some other sorts of yacht owners.
Matching by eye can be very difficult, in my experience. I got a 3-page Awlgrip color chip brochure (for free!) from Defender, and used a 1/4" hole punch to put a hole dead-center of each chip. By placing the brochure flat on any area of interest on the boat, I found it to be much easier to discriminate colors with the area of interest entirely surrounded by the chip color, versus a side-by-side comparison.