Hmm, I think it must be due to the Windows basic colour settings.
My colour picker background is white with black text.
It might be helpful were he to post a screenshot of a forum page.
(or even his desktop)
Unfortunately there are no screenshots so it is very difficult to understand the problem.
However, after changing the background colour and SAVING, it is necessary to switch to "Approach Mode" then back to "Airport Mode" to see the result.
The default flatten is in the CVX file.
In ADE, you can add a polygon with attribute "Exclude General" > "Airport Backgrounds" to remove the default flatten then add one which matches your photo scenery.
However, you will also need to modify the default water poly (which is not so easy)...