Okay, so first:
Light can be [is] broken down into a spectrum ranging from shortwave lengths to long wave-lenghs. Visible light (the part of the spectrum we humans can see) contains the colours which are all around us. An objects colour directly relates to which wavelength(s) of light it reflects.
White cars are best at refracting light as it reflects every colour in the visible spectrum (white light [like sunlight] being made from all the colours combined); black on the other hand doesn't reflect any colour light which hits it at all.
Since non-reflected light is absorbed into an object (in the cars case here) it can be converted into heat energy, warming the cars interior.
So the best colour car to get (to keep the interior cool) would be a hue which reflects the most sunlight (or absorbs the least depending on which way you look at it) of which would be white or silver in highly reflective/glossy sheen.
To make the interior of a vehicle warmer in climates which don't boast high temperatures all year round though, a colour such as yellow or grey in a non-metalic/shiny sheen is a good option- these colours aren't full extremes of black or white and so adequately reflect light whilst also absorb a fair amount too helping maintain a good temperature inside the car. Having a non reflective paint means less light is refracted and so more is absorbed into the vehicle- making it easier to warm up with less sun.
Interior fabrics/leathers mostly absorb heat and so the same principal applies: the darker the stuff, the hotter it will get. White interiors do heat up fast too if they are in constant sunlight with no way of getting cool (such as an open window or A/C. So putting your car undercover when parked or having one of those thingys which goes over the windscreen is a good idea in any situation.
A cars paint job though isn't really the biggest dictator on how hot a car will get- though it does place a good part; things such as windows, their tinting, other inner materials [plastics or metals] and the cars body itself also are factors in how much a car is heated and how fast, so all of these need to be taken into account before buying- as well as the overall appeal of the vehicle and whether it can fit anything bigger than a fat guinea pig inside...