Uuuuum YES it is!!
The deficiency doesn't CAUSE the fading, but rather allows the fading/sunbleaching to occur.
Genetics also come into play as there are two types of black gene, fading and non fading (jet black), its the fading types that benefit from the right dosage of daily copper. No amount of sun can bleach a horse with the jet black gene.
So yes the browning/oranging is sunbleaching caused by a deficiency in copper
The darker the horse (regardless of colour) the more copper they need.