Fifty Plus (50+) - Color-Matching Brake/Gear Cables to Frame Color

Are there good online sources, that offer ... and display online ... a wide selection of colors that brake & gear cable housing come in?

Heat shrink tubing is what you want. It comes in all colors and is easily applied. Just slide it over your cable casing and heat it. It comes in various sizes from small enough to use on spokes to large enough to use on your frame. The shrink-wrap you would use on your frame has glue strips and is flat. You just pull off the paper strips and roll the shrink-wrap around the frame pressing the glue strips together and then heating to shrink it tight. It's great for protecting the right chain stay and would match the colors used on the cable casing and spokes. You can print labeling information on it as well. If I were a bicycle parts supplier I would make this stuff available to bike shops for retail sales.
http://cableorganizer.com/heat-shrink/heat-shrink2.htm
http://images.cableorganizer.com/heatshrink/colored-heatshrink2.jpg

This search at AEbike shows a number of Jagwire products:

http://tinyurl.com/2e7kk9

Here's the source, better pictures:

http://www.jagwireusa.com/bulkhousing.html

The stuff is not cheap, another source is eBay where sometimes they sell it in more modest amounts. Branford sells peices for about one bike's build.

http://tinyurl.com/yr3r3b

Post this on the bike mechanics fourm. You will get many responses. Make sure you check out if the cable is for indexed shifting. There is a ton of old Schwinn cable out there - in many colors - that will not work with index shifters. Good luck.