You are correct that you cannot set fg separately for the miniplayer because that is (currently) shared with the main player panel.
(you could post a wish for dedicated elements for it)
But you could change the background colour: <element id="PlayerFlat.Background" bg="168,193,227"
That one is also shared between the mini player and the main player panel, but since you are using a bitmap skin, it's value is irrelevant for the main player.
The most actual and complete list of elements is probably my Sample Skin.
I try to keep that one updated when new elements are introduced, and prune the elements that I am (close to) certain about they don't function anymore.
A complicating factor is that there are elements that will not show up whatever colour you set for them, but if you then just remove them from your skin file, suddenly strange things happen.(Former member redwing used to do good work in posting and gathering new skinning elements that you can find here:
But regrettably he chose to disappear a while back, taking (and deleting) some useful contributions with him.)
In the past I created a (ridiculous complex to create t.b.h.) Excel tool. You could use it to load two different skin.xml files in it, and it would then show all differences.
The idea is that it has a 'master skin' xml included, and you could match that against your own skin to see what is missing from your skin.
I haven't looked at it for a while, but I think you should be able to find it doing a forum/google search.
If you are interested in it, I could take a fresh look at it and check it and update it with some more recently added elements.