You should read a bit about video standars and chroma subsampling.
The native pixel format of DVD and Blu-Ray videos is YCC 4:2:0 12-bit (8+2*8/4 - one full resolution luma and two half resolution chroma channels). This pixel format isn't supported by HDMI (<=1.4 ?and up?), so the player have to resize the chroma map in one direction: YCC 4:2:0->4:2:2 before it can send it out on HDMI.
This chroma resize step can produce high bit depth results (even floating point numbers, depending on the algorithm), so you can use the extra precision of DeepColor.
If you have native YCC 4:2:2 material (test and/or demo video in a prototype test/demo player) then the conversion between 8 bit and 8+ bit (/ luma channel) should be absolutely harmless. (Of course, bugs can occur and the DeepColor option in the OSD can trigger other hidden "features" too.)
"DIY certified hobby-calibrator" (based on ChadB's "warning signs" list