Hi Hallvard,
An example, quick rough, but may be good for starting out.
Since this is 'non-standard', it's slightly advanced and so it involves a few markup concepts that are just a bit above beginner level. But have a good read up in Markup in the Notation Reference, and I would be happy to help you out with any problems, as would many others I am sure.
Andrew
====
\version "2.19.81"
\relative c'' {
a1
% place in bottom layer
-\tweak layer #-1
-\markup {
\with-dimensions #'(0 . 0) #'(0 . 0)
% specify color
\with-color #(rgb-color 1 0.5 0.5)
% specify size
\filled-box #'(-2 . 6) #'(0 . 5) #0
}
a
-\tweak layer #-1
-\markup {
\with-dimensions #'(0 . 0) #'(0 . 0)
% specify color
\with-color #(rgb-color 0.2 0.5 0.5)
% specify size
\filled-box #'(-2 . 6) #'(0 . 5) #0
}
a
-\tweak layer #-1
-\markup {
\with-dimensions #'(0 . 0) #'(0 . 0)
% specify color
\with-color #(rgb-color 0.2 0.3 0.6)
% specify size
\filled-box #'(-2 . 6) #'(0 . 5) #0
}
a
-\tweak layer #-1
-\markup {
\with-dimensions #'(0 . 0) #'(0 . 0)
% specify color
\with-color #(x11-color 'DarkRed)
% specify size
\filled-box #'(-2 . 6) #'(0 . 5) #0
}
}
====