Colours are random as far as I can tell. No two or more intersecting paths can have the same colour.
Circles are a set distance apart. Not sure what this exactly is ?
Colour splits of circles are determined by line of sight. Standing in the centre of a room you would be able to most if not all of the circles depending on room layout but standing in the corridor or hallway you would not see the corners etc . The number of intersecting paths determine number of colour splits of a circle.
Have a look at Sundance's thread about the Foix Bunker for a good example - rooms with alcoves/dividing walls will have more than one path colour in them eg Sleeping quarters depnding observation point but open rooms like the office will have only have one basic colour. Then you have to remember about staning outside looking in. You can suffer from path bindness looking at a map for too long - it is an idea to get some proof readers to check you've not missed any paths.
If you're planning on making each piece/room random like Space Hulk / Doom tiles your best bet would be to make the transparent overlays with open circles that you can colour yourself with overhead projector dry wipe pens using OHP transparencies you've preprinted and then cut to fit each room. The only problem with this is it makes initial setting up very long winded - the reason I gave up on this idea - players get very board and frustrated waiting or you spend a long time setting up before they get there and you usually miss some path somewhere - I know I did.
Miah999 created some block tiles based on Avalon Hill's Betrayal At House On The Hill and Rackham's Hybrid which are available on Plageman's Black Sheep Archive blacksheep.eclipsenoire.net/main.php which is probably an easier way to go.
Hope this helps in someway and I've not put you off too much.