My MacBook Pro, gets stuck at boot screen and the graphics look weird I have a 2011 MacBook Pro i7 and I have upgraded the memory to 8 GB and the HD to an SSD 500GB. The other day I had it running for 3 days non-stop and when I tried to use it it was frozen. I reset it and when it went to the boot screen the graphics looked funny like 16bit color mode, and it got stuck after a couple of seconds. I removed the hard drive and I noticed that I can access all the files and I backed them up. I downloaded Yosemite and I tried to install it from an USB stick but after loading for a while the screen turned gray and it froze. I'm a bit baffled as what might be the problem? Do you think it's the hardrive or is the hardware failing? I'm attaching screenshots of the bootup srceens.
Unfortunately you may have hardware issues if yours is an "Early 2011" model, many owners have reported symptoms similar to yours with the culprit being failed ball grid array solder on the graphics processor. There's some info here. The article has a link for a utility that will allow you run solely on the integrated Intel HD3000 graphics so at least you'll be able to continue using your MacBook (in theory at least, we do have an Early 2011 i7 15" on campus but so far it's not had any problems).

Ultimately the fix will either be a replacement logic board or you can try any one of a number of companies offering to "re-ball" or "re-flow" the solder on GPUs. Any company offering Xbox 360 or PS3 repairs should be able to do this. 

Here is a link to the "MacBook Pro Repair Extension Program for Video Issues" for your Mac model.