Yellow is certainly similar to FAKE. Basically, instead of them being cops, they are snatchers xD! The relationships are also VERY similar. The art is gorgeous, the storyline is fine, and the characters from both are like twins =3! But warning, like in the last volume of FAKE, there is yaoi o.o
Yellow is like FAKE retold. In FAKE, one character is bi and he loves his partner who's straight (or, more appropriately, confused and/or doesn't realize it). In Yellow, one is gay, and you know the rest. There's a difference between the jobs (in FAKE, they're cops while in Yellow, they're 'drug snatchers' but what they do is pretty similar - they're sort of detectives ;^^) (Only, I found FAKE much better.)
Both manga are short. W/A is only 6 volumes and YELLOW is only 4. Both manga deal with drugs and yaoi undertones. YELLOW is explicit. W/A is definitely more implied but any yaoi-fan will be able to see and appreciate the beautiful relationship development.
Both are well-developed yaoi mangas with lots of action and mystery. Although "Yellow" is slightly more exciting and "Koujitsusei no Tobira" seems more like a slice of life story, at least in the beginning. But both are a very enjoyable read with a captivating plot.
If you like the seme having to WORK for the affection of the uke, instead of the "short and sweet" romances, then Crimson Spell and Yellow are the ones for you ;)
(also had some GREAT humor, and pretty schmexy stuff thrown in ;)