Looking for an RGB color table, constant intensity? Answered

Putting aside the intensity question, part of the problem is that generating a typical "rainbow" of perceived colors, where blue/violet is cold and red is hot, maps to RGB in a fairly complicated way.

If you want violet (R+B) to be coldest, then blue, then green, yellow (R+G), orange (2R+G), red, you need to run the intensities something like this:

R: 1 . . - . . 0 . . . . . 0 . . - . . 1 . . 1
G: 0 . . . . . 0 . . - . . 1 . . - . . - . . 0
B: 1 . . . . . 1 . . - . . 0 . . . . . . . . 0

Once you've got the relative proportions, then you need to do some overall scaling of each color to make the intensities consistent to the eye (in particular, scale down the green relative to the other colors, as Steve mentioned).

Doing this "correctly" is a fairly complicated job, and is different for pigments than for LEDs. A Google search for color temperature chart may give you some help in fabricating a solution.

You'll see a huge change in intensity because of the photopic sensitivity of the human eye - we don't see red and deep blues anything like as well as green,

1. Set up two delay counters Blue and Red.
2. load 90 into blue-on and 10 into blue-off or red on. 
3. simple takea blue bit away and a bit to the red.
4. Count down 89 x 10us and blue off turn red on for 11 x 10us
5. Decrement a big loop counter
6. Test for big loop zero and B ==> R shift Step_3
7. Now red off and blue on  as Step_4

• start with blue 90%PWM on
• then start red 10% PWM on
• Next add a pulse to red and take away a blue pulse
• continue until red is 90%PWM on and blue is 10% PWM on
• play with the big loop timing between cycles to taste.

You need to be logarithmic -
The first 10% will appear as about 40% total intensity
output 50% will be about 90% visible intensity
The last 50% will be the last 90-100% intensity.

Your eye iris will close as it gets brighter, making everything after 'perceived' seem the same brightness.

I have an arduino sketch I wrote that puts 255 'constant' brightness rainbow values into an array, then you reference that array later for values (lookup is faster than doing the math in my application). I can post code later when I boot up my laptop.
http://www.youtube.com/watch?v=tWjcMlhfz1Y

Tricky to calculate, as red, green and blue LEDs have widely varying intensities for the same current.  The eye's sensitivity to each colour also comes into play.
When I was making a PWM driven RGB circuit I used 3 different series resistors to produce white at 255 setting.  Values lower than 255 then gave a pretty good intensity match.  I think I've still got the circuit made up, but can't check now.  I'll tell you the values I used tomorrow if you're interested in this approach. (Mine was a Picaxe rather than Arduino, but the principle's the same.)

Let me start at the beginning. Making a weather station. Want to represent the temperature outside with a colored light. When it is cold, I want the light to be blue, and when it is hot, I want red.