As of Excel 2010, I believe this is no longer correct - you can use the Range.DisplayFormat property to access the actual displayed .Interior.Color or .Interior.ColorIndex, including conditional formatting, and including the newer conditional formatting not covered by Rick's code (e.g. color scales). However, it still does not function correctly in a UDF, and obviously Rick's code is backwards-compatible and offers additional utility. I only figured this out myself when I needed to put something together for someone who loves color scales.
Microsoft has not done a great job of publicizing this, to put it lightly.
https://www.youtube.com/channel/UCnxwq2aGJRbjOo_MO54oaHA
https://www.youtube.com/watch?v=SIDLFRkUEIo&lc=UgzTF5vvB67Zbfs9qvx4AaABAg
https://www.youtube.com/watch?v=v_1iqtOnUMg&lc=UgxLtKj969oiIu7zNb94AaABAg
https://www.youtube.com/watch?v=f7xZivqLZxc&lc=Ugxq4JHRza_zx3sz0fx4AaABAg
https://www.youtube.com/watch?v=f7xZivqLZxc&lc=Ugxq4JHRza_zx3sz0fx4AaABAg
https://www.youtube.com/watch?v=f7xZivqLZxc&lc=UgzMCQUIQgrbec400jl4AaABAg
https://www.youtube.com/watch?v=LuAipOW8BNQ&lc=Ugz0Uy2bCSCTb1W-0_14AaABAg
https://www.youtube.com/watch?v=ITI1HaFeq_g&lc=Ugx12mI-a39T41NaZ8F4AaABAg.9iDQqIP56NV9iFD0AkeeJG
https://www.youtube.com/watch?v=vXyMScSbhk4&lc=Ugxa2VYHMWJWXA6QI294AaABAg. 9irLgSdeU3r9itU7zdnWHw
https://www.youtube.com/watch?v=tPRv-ATUBe4&lc=UgzFkoI0n_BxwnwVMcZ4AaABAg
https://www.youtube.com/watch?v=LuAipOW8BNQ&lc=Ugz0Uy2bCSCTb1W-0_14AaABAg.9htChVuaX9W9htG01cKBzX
https://www.youtube.com/watch?v=LuAipOW8BNQ&lc=Ugw6UrV69zpeKvLOeOV4AaABAg. 9ht16tzryC49htJ6TpIOXR
https://www.youtube.com/watch?v=LuAipOW8BNQ&lc=UgwMKwGZpDjv7vi7pCx4AaABAg
https://www.youtube.com/watch?v=LuAipOW8BNQ&lc=Ugw6UrV69zpeKvLOeOV4AaABAg. 9ht16tzryC49htOKs4jh3M
https://www.youtube.com/watch?v=LuAipOW8BNQ&lc=UgxVW-am20rQ5GFuJ9F4AaABAg
https://www.youtube.com/channel/UCnxwq2aGJRbjOo_MO54oaHA