10 Changes Doctor Who Hoped You Wouldn't Notice

10. That Random Rainbow Regeneration

After being a staple of the classic era for many decades, the Master first popped up in NuWho during the Series 3 finale. But then - in a shock twist - he was shot dead, forcing him to regenerate inside the Tenth Doctor's TARDIS.

It's an iconic moment in the show, but something that many people don't pick up on here is the fact that the Master's regeneration energy looks totally different to the standard golden colour we've come to expect from NuWho regenerations.

Instead, the Master's regeneration has noticeable streaks of blue and green colour added into the mix, creating a striking rainbow effect that has only been used once since NuWho began.

Every other regeneration we've seen over the last 16 years (from the Doctor, to the Time Lord General, to Melody Pond) has copied the basic golden look that was established by the Ninth Doctor's regeneration at the end of Series 1.

And so, we have to question why this one-time rainbow regeneration happened. Oddly, former showrunner Russell T Davies is on record as saying that he wanted the Master's regeneration to echo the Ninth Doctor's, so what's with all the colours?

There are many fan theories - given that this is only the second regeneration in modern Who, some believe that the overall look hadn't been finalized yet, despite Davies' comments - but given that the colourful style was a one-time only change, our best guess is that it was simply a failed experiment, something that the showrunners then decided to sweep under the rug.