Is there a way to get the current colour of the image_button tile
No - the visual appearance of a DCL image/image_button is one-way: you can modify what is displayed, but you cannot query the properties of the tile to obtain information about what is currently being displayed.
Is there a way to toggle the colour to a new different one?
I would suggest initialising the image_button to a known colour (either in the DCL file or better yet, in your program), and assigning such colour to a variable which may then be referenced in the action_tiles statements for your dialog. Ensure the variable value is updated when you alter the colour and you can easily toggle the colour.