I would recommend a soft gray...see photo below...

I am assuming you haven't finalised your wall colours and colours of any other fixed elements. In case you have finalised your color scheme you would need to see which of these colours blend well or complement with that scheme. Assuming none of this info is available, then you could look at the neutrals which go with all colours. The white tile and the 2 colours at the end would be preferable. If you want color on the floor as well as drama, you can choose from the 2nd options from both sides or even the 1st from left.

beige or brown

Abcd XYZ , How much is the difference in the floor level for each room. Frankly I could not understand your plan of using the tiles for levelling the gap. Can you please elaborate a bit more on how you plan to close the height difference using tiles. Tiles are usually 8mm thick and any more difference would be difficult to level. Tiles are most brittle at the edges and if you plan to leave them like that soon you may have ugly broken edges at door entrances. Thanks, Nitin Cornerstone Designs, Bangalore.