colours.php

<?php

function colours($red, $green, $blue) {
    if ($red > $green) {
        if ($red > $blue) {
            return 'red';
        } else if ($blue > $red) {
            return 'blue';
        }
    } else if ($green > $red) {
        if ($green > $blue) {
            return 'green';
        } else if ($blue > $green) {
            return 'blue';
        }
    } else if ($blue > $green && $blue > $red) {
        return 'blue';
    }

    return 'grey';
}

$tests = array(
  array(3, 2, 1, 'red'), 
  array(3, 1, 1, 'red'),
  array(3, 1, 2, 'red'),
  array(2, 3, 1, 'green'), 
  array(1, 3, 1, 'green'),
  array(1, 3, 2, 'green'),
  array(2, 1, 3, 'blue'), 
  array(1, 1, 3, 'blue'),
  array(1, 2, 3, 'blue'),
  array(1, 1, 1, 'grey'),
);

foreach ($tests as $test) {
    if (colours($test[0], $test[1], $test[2]) != $test[3]) {
        echo "WRONG ";
    } else {
        echo "RIGHT ";
    }

    echo implode(', ', $test) . "\n";
}