function randomcolour()
{
colour_array = new Array();
colour_array[0] = '#D41E46';
colour_array[1] = '#FFE15F';
colour_array[2] = '#D023AA';
colour_array[3] = '#101010';

<!--- THE ABOVE ARRAY CAN BE ANY LENGTH AND CAN USE HEX VALUES INSTEAD OF COLOUR NAMES --->
count = colour_array.length;
random = Math.floor(Math.random() * count);

document.getElementById('maintable').style.background = colour_array[random];
}
onload=randomcolour;