Likewise on old (pre-privatisation) London Connections maps, British Rail (
a.k.a. Network Southeast) lines were shown as orange.
Later, on London Connections maps the Silverlink TOC's routes were shown as orange lines - London Overground took over the Silverlink Metro routes in 2007.
Worth stating that the orange of London Overground is a slightly different shade to what the ELL orange was.
I think essentially orange was chosen because it was the colour that was available for use on the Tube map.
(The two oranges, ELL and LO, did very briefly co-exist on the Tube map for just over a month in 2007 - as can be seen
here.)