Clee is correct, I used a tinting spray and dusted the inside of the covers lightly, a number of times till they were the same shade. I did however do both (one more than the other to even up the shade) and the finish is obviously darker than standard but in my opinion looks better. It has a meaner appearance than before. I didn't want it to look chavvy or aftermarket but think the result was better than I expected. The spray is removable with thinners so nothing lost if it hadn't gone to plan. But the covers were too different so in my eyes, I had no choice.
So this inspired me to get my spare 'pair' of rear lights off the shelf and clean them up. Having removed the covers, I find that these are completely different colours.
May be red cars had tinted perspex covers... Anyway, the tint spray is on its way.