Reasons to be careful about color:
1. Historic tartans were not mass produced. Dyes and manufacture were subject to variation. There is no precise record of the original tartan colors. Those who have researched historic dyes, and spun and woven their own fabric may have the best feel for what the original colors might have been.
2. Modern dyes are different from historic dyes.
3. Is the stereotype of the thrifty Scot a complete myth? Would you throw away a perfectly good tartan if it were somewhat faded? Anybody answering "yes" who is in possession of heirloom tartans may contact me. Please!
4. You've heard of perfect pitch, but have you ever heard of perfect color? Perfect pitch exists, but I've never heard of evidence of perfect color. Color perception is an inexact science. Some people have a greater ability to discriminate between colors than others. So far as I am aware, "perfect color" is unknown in human experience, while perfect pitch is rare but is documented.
5. The light which illuminates a tartan can change the appearance of its colors. Should its colors be evaluated by daylight on a cloudy day, or a day with a clear blue sky, or perhaps by the light of a smoky peat fire?
6. The colors next to a color can strongly affect your perception of those colors. For example, most of the red stripes, coded as red=255, green=0, blue=0 in my definitions, appear as more orange than red. To be orange, they would have to contain green, too. The green from the adjacent stripes may "mix" visually to make the red seem orange. This effect seems most pronounced where green and red stripes cross to produce a green-red "checkerboard". This effect seems much more pronounced on my monitor than it does on the actual cloth.
7. Computer monitors, when used to display colors, are subject to variations. They are adjustable. Only graphics professionals generally have equipment required to calibrate their monitors to reproduce color accurately.
8. Printers, film transparencies, and photographic prints all have different and varied output. So also do looms!
9. Computer monitors use different primarary colors. They're additive. Red, Green, Blue. Pigmented colors are subtractive. Rather than adding light, they absorb it. So it's very hard to present a good saturated forest green found on some of my tie "swatches". They are dark, but colorful, in a way that is difficult to show on the monitor.
I have adjusted my color values to conform to a few rules, but you could easily use other rules.
My rules are generally that color intensity numbers generally belong in binary orders of magnitude.
0 - totally dim - like me in the morning!
255 - totally bright - like me at my best!

Generally, for the photographers out there, color intensity is a lot like f-stops for light intensity.
My Tartan Java applet doesn't want the name of a color, it wants the red, green, and blue (RGB) values. While more difficult, this allows for the use of more colors than you could easily name!
Each color has a value from 0 to 255.
You can use the custom color tool in windows paint, or some other tool to help you find the RGB values of a particular color.
However, what matches on your monitor may not match just the same on another monitor.

red, green, blue color
255, 255, 255 white
0, 0, 0 black
192, 192, 192 light gray
128, 128, 128 gray
64, 64, 64 dark gray
255, 255, 0 yellow
255, 128, 0 orange
64, 0, 0 brown
128, 0, 128 purple
0, 255, 255 cyan
255, 0, 255 magenta