As far as I know colour is not available at all for dialogues unless you use a DLL designed for the purpose. There's one called MDX.DLL but I've never used it so I don't know it's feature set.
As for 'tables' that is just the name the dialogue is called when called in a remote script.
dialog [color:red]name[/color] {
The bit in red is the table.
From the mIRC help file:
The dialog table
You can use the dialog prefix to create a dialog table called name in a script using this format:
dialog [-l] name {