Ileana114 - if your car is equipped with a factory installed HID system you need a D2R bulb
. And yes it is best to replace the bulbs in pairs otherwise the bulbs will not match as one bulb is older and already burned in.
All cars that come with a factory equipped HID system use 4100k-4300k bulbs. If you want a stock look I would recommend the Philips 4300k D2R
If you want something with a little but more color there is also 5000k, 6500k and 8000k. Keep in mind that the higher you go in Kelvin (K) the more color but the less usable light.
I would personally recommend 4300k if you want to maintain the stock HID look and to get the most light output. 5000k is also nice and will give you more of a crisp white color with a very slight hint of blue.
Here are some pictures from one of our customers that installed our 5000k bulbs -