I started replacing the 42mm and 44mm festoon-type dome light bulbs in my car with LEDs years ago (most of them from DX) and it's sort of an ongoing process because they have a tendency to break. Since they're cheap and easily replaceable, I don't care too much about reliability. Most of the blue ones aren't too bright (my car has a blue speedometer and radio backlight with red ambient lights) so I ditched blue and went back to white. The ones I'm using right now are these T10s which aren't all that bright but work well in tight spaces, these 42mm LEDs in my dome lights (brighter than most SMDs I've tried; I can't speak to their reliability yet, because I've only had them for two months) and these T10s where size isn't a concern (see here for measurements; not mine).
There is a good chance that most LEDs will have a faint glow even when off, depending on your car and how it's set up (when you turn the key, most systems in a car that uses a CAN bus aren't truly "off" until the central electrics control module shuts them down). In most modern cars, you'll need to use LEDs with a small resistor if you want them truly off until you need them. If the faint white glow doesn't bother you, you don't really need to worry about it since the LED's power consumption is negligible. Personally, I rather like it.
This thread is more than a year old but I wanted to share my experience.
I will only talk about festoon light.
First I bought this
The main advantage that it have the same size as a normal festoon but not really bright and very cool white.
then I come to this
I don’t know what kind of led it is, provided a good amout of light, but still very blueish. It replaced the above for the trunk light for a long time.
For dome light, it take 31mm festoon so the choice was limited. I ended up with this
very bright, but the color was not very good, I think that the CRI was very low.
Then I discovered festoon with Cree XR-E.
The metal ring show it’s a XR-E emiter. The same emiter used on many single AA powered flashlight.
This is brighter that any festoon led I tested before, very cool white but not blue, with proper CRI so good color rendition, better than SMD used on other festoon.
I didn’t test on car that have OBD check but on one car I used it, the light is dimmed before shut off completely. With that led, it dimmed to but it flicker a little.
The con is that it’s a little more expensive than other led festoon.
I am sorry all my link are on dx.com, I am not affiliated with them. I think that those xr-e festoon can be found on aliexpress or ebay.