I'm not sure I understood your question, if it is: "Why the two Optionmenus behave differently?" then my answer is:

The 'brscmd' sub dynamically creates and destroys an Optionmenu in the active cell, the second Optionmenu is fixed and created only once in the cell with index "4,4".

It's good practice to "use warnings" and follow it's recommendation (Scalar value @colour_list[0] better written as $colour_list[0]). Also quote 'custom' in the @colour_list array and "use strict" (without "vars").

Best regards, Stefan