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15 years ago
I had the same problem as heavyraptor2 so i made this function...
<?php
function imagecolorize($im,$endcolor){
    //funcion takes image and turns black into $endcolor, white to white and
    //everything in between in corresponding gradient
    //$endcolor should be 6 char html color

    //make sure it has usable palette
    if (imageistruecolor($im)) {
        imagetruecolortopalette($im, false, 256);
    }

    //first make it gray to be sure of even results (thanks moxleystratton.com)
    //comment this loop if you want the output based on, for example, 
    //the red channel (for this take a look at the $gray-var in the last loop)
    for ($c = 0; $c < imagecolorstotal($im); $c++) {
        $col = imagecolorsforindex($im, $c);
        $gray = round(0.299 * $col['red'] + 0.587 * $col['green'] + 0.114 * $col['blue']);
        imagecolorset($im, $c, $gray, $gray, $gray);
    }

    //determine end-colors in hexdec
    $EndcolorRGB['r'] = hexdec( substr($endcolor, 0, 2));
    $EndcolorRGB['g'] = hexdec( substr($endcolor, 2, 2));
    $EndcolorRGB['b'] = hexdec( substr($endcolor, 4, 2));

    //determine gradient-delta's
    $stepR = (255-$EndcolorRGB['r'])/255.0;
    $stepG = (255-$EndcolorRGB['g'])/255.0;
    $stepB = (255-$EndcolorRGB['b'])/255.0;

    //aColor contains the 256 gradations between endcolor(i=0) and white(i=255)
    $aColor = array();
    for ($i = 0; $i<=255; $i++){
        $aColor[$i]['r'] = $EndcolorRGB['r'] + ($i*$stepR);
        $aColor[$i]['g'] = $EndcolorRGB['g'] + ($i*$stepG);
        $aColor[$i]['b'] = $EndcolorRGB['b'] + ($i*$stepB);
    }

    //for every color-index we now replace $gray-values for $aColor
    for ($c = 0; $c < imagecolorstotal($im); $c++){
        $currentColorRGB = imagecolorsforindex($im, $c);
        $gray = $currentColorRGB['red'];//image is grayscale, so red,green and blue
        // should be equal. We use this number as key of aColor
        imagecolorset($im,$c,(int)$aColor[$gray]['r'], (int)$aColor[$gray]['g'], (int)$aColor[$gray]['b']);
    }
}?>
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