I have been looking for a new trials tyre recently and have noticed that the white tires are considerably more expensive than the black ones. is there actually a reason behind this or is it because they are more sought after? 
I have always assumed it’s because tires are rubber, rubber commonly comes from oil, and oil is black.
So making a white (or non-black) tire means using something less common. Natural rubber maybe?
Grinding unicorns into paste is taxing work.
In black tires they use ash for the tire not becoming brittle. Ash is cheap and easy to obtain. For white tires they need other ingredients that are more expensive. But that’s only pennies. The major difference in price between black and white tires probably is based on the fact that for white tires the batches are much smaller.
Truth be known, true rubber is actually white in colour, but there likley is not a lot of real rubber being used in tires nowadays. Cost is probable due to limited batch runs as Yeti has mentioned.
Carbon black is the additive that most tires have as a filler. Wikipedia explains a lot about its use in rubber. It talks of fumed silica being added for non black rubber.
As stated above, batch size is probably the driving cost factor.
Black is also the best UV blocker to protect rubber and plastic to any depth. Any other color needs more UV additive.
I have to agree with the batch size theory. In years past, when colored tires were more popular (in BMX and other money categories), white tires cost about the same as black ones. Of course you could also get a range of other colors.
Funny, I just learned the other day about natural rubber being white. At least when it comes out of the tree that is… (taken in Costa Rica, near the Arenal volcano on Nov. 26.
Automobile tires (late 1800’s & very early 1900’s) were all white, but due to the fact that they got stained & dirty so quickly they soon started to be colored black (some were done in brown / tan) so that they would remain more or less a consistant colour.
One of the large expenses associated with the restoration of antique cars is born in the cost of the white tires that are being applied to them. Some are still being manufactured (on request) from the original molds that were in place well over a century ago.
I’ve seen some of those in museums; some say Coker on them. 
but why do they make larger batches of black?
black is more popular, because dirt is much more black than white, and black tires don’t show the dirt as much.
the more elemental question is: why is dirt black instead of white?