in using VCL
As with menus (menu items), an application can create an owner-drawn combo box to take responsibility for painting list items. An owner-drawn combo box can list/show information other than text strings - usually some graphical elements.
To demonstrate owner-drawing with combo boxes you'll create a combo box full of colors (two variations) and a combo with true-type fonts showing the actual image of the font.
Read the full article to learn about Drawing a TComboBox: Use Owner Drawing to create a Color Pick List and a Font Pick List.
Related: Owner Drawing Popup Menus | Implementing OnMouseOver for Items in a TComboBox | Professional Looking Grids with Custom Drawing