Here's something I whipped up. The CSS formatting is horrible but the amount of it in there was annoying me since it's not the point of the code so I packed it all together.

PHP Code:
<?php
    // declarations
    $cols = Array($_POST['colourone'], $_POST['colourtwo'], $_POST['colourthree'], $_POST['colourfour']);
    $colours = Array(); $final = Array();
    
    // remove # symbol
    foreach ($cols as $colour) array_push($colours, preg_replace('/#/', '', $colour));

    // nested for loop to only calculate midpoint between unique unordered pairs
    for ($i = 0; $i < count($colours); $i++) {
        for ($j = $i+1; $j < count($colours); $j++) {
            $rgb_one = getRgb($colours[$i]);
            $rgb_two = getRgb($colours[$j]);
            $rgb_three = getMidColour($rgb_one, $rgb_two);
            $triplet = Array(implode($rgb_one), implode($rgb_three), implode($rgb_two));
            array_push($final, $triplet);
        }
    }
    
    // returns array containing individual RGB components in hex
    function getRgb($col) {
        return Array(
            substr($col, 0, 2),
            substr($col, 2, 2),
            substr($col, 4, 2)
        );
    }
    
    // calculates the midpoint between two colours
    // double conversion is performed to convert from a string for arithmetic
    function getMidColour($rgb_one, $rgb_two) {
        $red = dechex((hexdec('0x'. $rgb_one[0]) + hexdec('0x'. $rgb_two[0])) / 2);
        $green = dechex((hexdec('0x'. $rgb_one[1]) + hexdec('0x'. $rgb_two[1])) / 2);
        $blue = dechex((hexdec('0x'. $rgb_one[2]) + hexdec('0x'. $rgb_two[2])) / 2);
        
        return Array($red, $green, $blue);
    }
?>

<html>
    <head>
        <title>Colour generator</title>
        &lt;style>
            body { background: #FAFAFF; font-family: Tahoma; margin: 35px; }
            td { border: 1px solid black; }
            tr.headings>td { border: 0px; font-size: 10px; text-align: center; }
            .swatchtable { border: 1px dotted black; padding: 10px; }
            tr.swatches>td { width: 70px; height: 50px; }
            .formdiv { float: left; border: 1px dotted black; padding: 10px; width: 500px; }
        </style>
    </head>
    
    <body>
        <?php if ($_POST['submit']) { ?>
            <div style="float: right;">
                <h3>Here is your colour table:</h3>
                <table class="swatchtable">
                    <tr class="headings">
                        <td>First</td>
                        <td>Middle</td>
                        <td>Second</td>
                    </tr>
                    <?php foreach ($final as $triplet) { ?>
                    <tr class="swatches">
                        <td style="background: #<?= $triplet[0] ?>;">&nbsp;</td>
                        <td style="background: #<?= $triplet[1] ?>;">&nbsp;</td>
                        <td style="background: #<?= $triplet[2] ?>;">&nbsp;</td>
                    </tr>
                    <?php } ?>
                </table>
            </div>
        <?php } ?>
        
        <br /><br />
        <div class="formdiv">
            <form method="POST" action="<?php echo $_SERVER['PHP_SELF']; ?>" name="colours">
                <h3>Enter four colours in hex:</h3>
                <input type="text" size="7" name="colourone" />
                <input type="text" size="7" name="colourtwo" />
                <input type="text" size="7" name="colourthree" />
                <input type="text" size="7" name="colourfour" />
                <input type="submit" name="submit" value="Submit" />
            </form>
        </div>
    </body>
</html>