Has anyone ever swapped out the standard bulbs on smiths gauges for different colours? If so, does anyone know the type of bulb i need? I had a look on the bulbs themselves but there's no markings i can see

Yep. Swapped mine for LED; take the bulb out and along to your local Halfords to determine the type in yours.

Ive just bought some 10mm leds for my rc truck and it struck me they would fit the gauges, have a look on ebay for rc leds and they come in white, red and orange.
Sure they are just normal leds that Maplins might have.

actually im not sure i can get the bulbs out the speedo and tach without taking the casing apart, which im not too keen on doing

the bulbs, (at least in triumph cars) are 2.2watt mes (miniature edison screw)
the bigger gauges have bulb holders with small spring clips, the smaller ones push in from behind.
the gauge colour is from the (paint) colour of the inside of the gauge

this link from our mgb friends should help you
http://www.mgb-stuff.org.uk/led.htm