CENTER makes most sense, because it's pronounced CEN-TER. CENTRE is stupid, because you don't say CEN-TRE.
The U in COLOUR is completely unnecessary.
Firstly, you need to remember that the English language grew from many different languages. French is one of those and "centre" is from there.
Unfortunately you live in a country which succumbed to the desires of one man to standardise your pronunciation of the English language into the Boston accent of his time, Noah Webster.
Hence your col-OR whereas in Britain, the commonwealth countries and most of the rest of the world, we tend to actually pronounce the diphthong, OU, in colour, the same goes for the rest such as armour, labour, etc.
So our crazy penchant for those extra U's is not a weird affectation, it is just that Webster's desire to allow Americans to be able to communicate clearly with each other made him make certain choices. The 'U' is necessary to the rest of the world, it is only unnecessary for Americans.
N.B. I am not attacking or being negative, Webster's work was amazingly effective and American standardisation of the language actually preceded such work in the UK.
P.S. Can you pronounce buoy without saying boo-ey? We can.
