A more informed source has now said that there will be no change to the 50p colour during 2012, but the colour may change next year.
Another source has said that at the end of this year the diamond-blue 1st class will be changed to post-office red. Does this mean more colour changes - and for what purpose?*
At this time the new postage rates have not been announced, and they are unlikely to be announced until near the end of this month, for implementation at the end of April or in early May, so this colour change would not seem to be caused by the tariff change. We'll publish the official explanation when we get it.
The 50p grey has not so far been reprinted in 2010/11 and so only exists from counter sheets with no year code.