I had the same black screen issue (rainbow square issue) and got it running. I spent about 50 hours trying different things, different images, different SD Cards, different config.txt edits and a whole
bunch of reboots and switching machines, surface testing SD cards, checking check sums. Heres what I finally did to get it running. First I looked up the timing table for my monitor user manual....the part that tells you what resolutions your monitor will do.
Then i edited the config.txt file on the Marshmallow SD Card from my windows 10 box. The parts I changed were only the Drive setting, the Group setting, and the mode setting. Raspberry has a
nice description of what everything is on their site.....I googled raspberry config.txt. I use a DVI monitor so I chose drive one, the timing table that mostly matched the one from the monitors user manual was group 2,
and for mode I chose the mode that matched my monitors highest resolution...in my case it was 1440x900 at 75mhz....looked it up on the mode table from rasberry. Mine was mode 48 1440x900 at 75mhz. Mode and group are tied together so if you choose group 2 you use the timing table for group 2 to get the mode. That being said all this did not work!!!
50 hours of failure was pretty frustrating......and just by accident I switched power supplies to my rasberrry....from 5v at 3amp to a regular 5v/2a (stock walwart) and it started working.
So what fixed it? Have no idea. Switched power supply and it worked. But dont know if it was something else I did so I included my changes. Also theres a little delay where it says it cant find the signal before it kicks on, its like 10 seconds or less.
Theres my 2 cents and Im a noob...just stubborn....and I know more about linux now than I ever wanted to.
Cheers