Hi Sly,
Typically in internal combustion engines, blue smoke is a sign of the engine burning excess oil, and white smoke is a sign the engine is burning coolant. Now a lot of different things can cause these problems. Assuming your Defender has higher mileage, these are what I think could be causing it:
Blue Smoke: Worn valve seals, worn piston rings, worn engine oil seals, or head gasket failure.
White Smoke: Warped cylinder head, cracked engine block, or head gasket failure
Now some white smoke at startup is normal, especially when it is cooler outside. That is usually just steam coming off from the condensation inside the exhaust pipe.
Because you have both kinds only at start-up, I think it could be a head gasket starting to give out. I would have your mechanic check it out sooner than later.
Best Regards,
Drew.