How can you find reddest colour in a fixed colour display?

I am writing a UI for a legacy computer called an Amiga (some of you may remember these). They had display modes that had fixed numbers of colours - 2,4,8,16 etc. and you could set any of these using the RGB of 0 to 255.

So on a 4 colour display you could have:

Colour 0 R=0, G=0, B=0 (black)
Colour 1 R=255, G=255, B=255 (white)
Colour 2 R=128, G=128, B=128 (mid Grey)
Colour 3 R=255, G=0, B=0 (Red)

The maximum number of colours you could have on 'normal' display was 255 (there were some special display modes that pushed this higher but I don't need to worry about these).

I am trying to write some C code that will read through the display's colour list and find the reddest, greenest, and bluest colour, but I just cannot get my head around the comparisons involved.

If I have 3 vars -red, green & blue - all ints which hold the current colours rgb vals and 3 more - stored_red, stored_stored_green & stored_blue.

How can I write a function that will remember the colour number which has the most red in relation to the other colours?

I've tried:

If ((red>stored_red) && ((blue <stored_blue) || (green<stored_green)))

But this doesn't work. I think I need to work on ratios but just cannot figure out the maths.

Solution

There are different ways to check for maximum "redness"/"greenness" etc, this first simple way here is what my answer originally used:

/* for all palette (color table) entries */
if(red > stored_red) {
    max_red_index = current_index;
    stored_red = red;
}
if(blue > stored_blue) {
    max_blue_index = current_index;
    stored_blue = blue;
}

and the same for green of course.
This will just give the colors with the maximum components, but after the comment of @Chris Turner I think this is probably not what you want.

Another way could be to check the ratio of red to the other colors (I'll just use only red from now on):

redness = red*2/(blue + green + 1)

This gives a number between 1 and 510 for "redness", it does not consider brightness. For example R-G-B 50-10-10 is more red (4) than 255-100-100 (2).

Many other formulas are possible, something like

redness = red*red/(blue + green + 1)

would also take the brightness into account and would consider 255-100-100 more red than 50-10-10.

To use these formulas, just set the red variable from above to the result of the redness formula.