What happens if we put 10bpc video in 8bpc composition?
10-bit footage will look the same as properly shot 8 bit footage, it's how it copes with grading and effects that makes the difference. If you have 10-bit video that's a few stops underexposed you still have a lot of information in the shadows - four times as much information as the 8 bit. You won't see that extra info until you correct the exposure–particularly because most delivery formats are 8 bit, like internet, or broadcast television (but not cinema). The same holds true for over-exposure and colour balance, and any effect that works with colour information. Make the same changes in 8-bit footage and you'll just get mush, because there isn't the data, just like blowing up standard-def footage doesn't make it the same as footage shot in HD. 

To understand why, consider the R, G and B channels in an 8 bit image. Each channel has 28=256 possible values*. If your footage is underexposed and the brightest highlight is at 50%, then you've only got 128 values per channel. A 10 bit image starts out with 210=1024 possible values per channel, and the same under exposed footage works out to 512 values, so it still has room to be graded and still have the same colour depth as properly shot 8-bit video.

If your project is set to 16 bit or 32 bit colour then After effects will use the full colour depth of the footage.

*this is simplified a bit, most video codecs are YUV rather than RGB, the information is distributed so there's more data for the Luma than the colour, and the exposure values aren't necessarily linear, but that's beyond the scope of this answer.