Generate colors between red and green for an input range

Question

Possible Duplicate:
Color coding based on number

I want for a user to be able to select from a range from 1-100, where as the numbers become less than 50, the color of the area becomes darker green, and as the color becomes closer to 100, the color becomes more red.

I am trying to make it so that as the range in more towards the center, the color should be close to white (where 50 = full white).

I tried the answer from here: Generate colors between red and green for a power meter? to no avail.... 50 ends up being a muddled green...

I have the following html:

<span><span class="value">50</span><input type="range" /></span>​

And the following javascript:

$(document).on({
    change: function(e) {

        var self = this,
            span = $(self).parent("span"),
            val = parseInt(self.value);
        if (val > 100) {
            val = 100;
        }
        else if (val < 0) {
            val = 0;
        }
        $(".value", span).text(val);
        var r = Math.floor((255 * val) / 100),
            g = Math.floor((255 * (100 - val)) / 100),
            b = 0;
        span.css({
            backgroundColor: "rgb(" + r + "," + g + "," + b + ")"
        });
    }
}, "input[type='range']");​

Fiddle: http://jsfiddle.net/maniator/tKrM9/1/

I have tried many different combinations of r,g,b but I really cannot seem to get it right.

Answer 1

You're getting the muddled green because of the way you're creating your gradient in RBG space. To get a "cleaner" gradient, you can use the HSV model as mentioned in the answer of the question you linked to.

^{RGB gradient (top) vs HSV (bottom)}

By scaling the H (hue) value between 0 (red) and 120 (green) you'll get a nice clean transition. However, at the mid point (60) you'll end up with bright yellow instead of your intended white. You can address this by modifying the S (saturation) value -- at 0 saturation, you'll end up with white (1 gives you full colour saturation.

Here's a crude example which scales the saturation from 1 to 0 and back to 1 as the input value goes from 0 to 50 to 100 - http://jsfiddle.net/xgJ2e/2/

var hue = Math.floor((100 - val) * 120 / 100);  // go from green to red
var saturation = Math.abs(val - 50)/50;   // fade to white as it approaches 50

p.s. Converting between colour models is easy using jquery-colors, although it's not too hard to roll your own.