You can find a list of the Drop Down properties you can access and modify here: https://www.mathworks.com/help/matlab/ref/matlab.ui.control.dropdown-properties.html
What is displayed in the dropdown is contained in the Items property. You don't have to loop though your array. You can assign them all at once. Just note that your array must be a cell array of character vectors, or a string array.
app.DropDown.Items = {'Red','Green','Blue'};
