Compared to the uploaded photo my young tiger centipede is now dark grey with only a small amount of orange-ish colour and its first maybe 3-4 segments are very stiff and are hard for it to move. I have now given it loads of burrowing space. I would love for some advise on how help it.
Unfortunately it's probably on its way out. The legs on the front segments have curled up, which is essentially them dying. Death is not exactly clear cut in centipedes, Some are paralysed from the front and stay alive for a long time, some recover. However I would not raise the humidity - this is a desert centipede and perfectly capable of shedding in relatively dry conditions. If you have been keeping it at high humidity it may well even be that this is what has caused this.
@BobBarley Ah OK, I assumed US hence Polymorpha, but you're probably right on species. As for humidity, I'm not saying high humidity is a problem for moulting, indeed it is necessary (really necessary for some species, less so for others) - rather the damage is done through long term stagnant conditions. Most enclosures, with their limited ventilation, are actually far more humid than we realise, even when the substrate appears quite dry. I up the humidity as I see a moult coming up out of worry, but then plenty of my centipedes moult without me doing that. I haven't lost a centipede to a failed moult in years, and my enclosures are pretty dry.