Hi,
You will have to program an ASPX page imitating the look, feel and
functionality of the respective dialog box.
Do you have any particular difficulties with programming such a page? I'd
start probably with a table representing the colors, and each cell would
have a script event attached for reacting on cell click. Then I'd probably
have an indicator of a currently selected color at the bottom - and a field
where color code is stored. Finally, the "OK" button would postback the page
with your current selection.