Okay i’m going to try to add this as an answer because I can’t do codes in comments:
<?php
if($_GET['colour'] && !empty ($_GET['colour']))
{
$colours= $_GET['colour']; //notice i pluralized.
}
?> //your code ended php, so i assume there is something here as it is not started again.
$meta_query = array('relation' => 'OR');
foreach ($colours as $colour) {
$meta_query[] = array(
'key' => 'colour',
'value' => $colour, //not sure why you have parentheses, but try with and without here i guess
'compare' => 'like', //switched back out of in because we are out of the array
);
}