Assuming the populate_Colours
method is in the global namespace, you may use the following code, which exploits both that all object properties may be accessed as though the object were an associative array, and that all global objects are actually properties of the window
host object.
var method_name = "Colours";
var method_prefix = "populate_";
// Call function:
window[method_prefix + method_name](arg1, arg2);