At the last MoT I got an advisory about the rear light not being very 'red' (along with 'noisy exhaust') and you can see why...
It's a cheap LED behind a clear white lense and while it looks red, it isn't a very full fat red - the number plate lights (tiny LEDs) are inside the light shining down. To try to make it more red, I've replaced the cheap one on the right with a 'CE' marked one on the left.
The results are very satisfying (if somewhat difficult to photo) as the whole light is now a full bright red, more bling !