PHP - calculate color (hex) based on given variables

Here's some stuff to get you started:

You can get the r,g,b values via:

$color1 = 0xfff000;
$r1 = ($color1 >> 16) & 0xff;
$g1 = ($color1 >> 8) & 0xff;
$b1 = $color1 & 0xff;

To interpolate between two values:

define('MIN', 0.10);
define('MAX', 1.00);
define('RANGE', MAX - MIN);

$i = 0.10; // importance

$i = ($i - MIN) / RANGE;

$r = $r1 + ($r2 - $r1) * $i;
$g = $g1 + ($g2 - $g1) * $i;
$b = $b1 + ($b2 - $b1) * $i;

Then you can put them back:

$color = ($r << 16) | ($g << 8) | $b;

But RGB isn't necessarily the best color space to work with. You might get better results by using something like HSL.

Alternatively to all of this, you could simply create 10 colors by hand that you like, and put them into an array:

$colors = array('#000000', '#100000', ... );
$color = $colors[intval(($importance - 0.10) / 0.0901)];

The 0.0901 number is simply (MAX - MIN) / 10 + smallDelta. The delta is used to keep the maximum index at 9 (instead of 10) for when $importance == MAX.