I inserted a new stereo bus to a project, but noticed that a soft synth track that was routed through it had its sound slightly colored, even though I had added no effects so far to that new stereo bus.
 
This was with the send level at 0.0, so I guess its just a doubling somewhere of volume right, as the sound is being heard in its normal soft synth track, and in the send track?
 
Then I tried a test with a reverb on a send and as an insert, and I couldn't get things to match at all.  The sound was different for example at 100% wet reverb on an insert as it was on a send at 0.0 (at 100% wet).     What's the logic to that, is it how it should be?

Your normal track out is to Master bus.
Add a new bus, route it's output to Master bus.
Add a send to the track, routed to the bus.  The Master now gets 2 copies of the track.
Add reverb plug to the bus, 100% wet, 0% dry.  Master gets dry from track, reverb from bus.
Add a send to each track requiring the same reverb.
Adjust the SEND level on each track to vary the amount of reverb (create a reverb mix) adjust the FADER on the reverb bus to vary the overall reverb level.
Make sure the send is set to POST so the reverb level follows the track level.

Thanks Karyn - yes, that's exactly how I understand it.    So why the difference in result with the reverb as a send, compared to an insert?
 
I made a test :   track with reverb as an insert played back, then same again using this time same level reverb as a send.   The result was louder when it goes via a send, all else being equal.   Why is that? 
 
And also when I route a track via a send that has the reverb disabled (ie no reverb) the result is still louder - it seems that there is the sound from the main track, plus the sound from the send track (even without effects) and the result is louder.   But its not just "louder" it sounds like 2 tracks together, so there is some kind of "thickening" of the sound (that is with no reverb) ... is that how its supposed to be?  

When you create a send to a bus you're splitting the signal, creating a second copy.  If the bus it's routed to is then routed to the Master (as it should be) the Master is now getting 2 copies of the dry track, so it will be louder.
 
Having a reverb in a bus with 100/0 (wet/dry) is the same mix as having a reverb in the FX bin (insert) with 100/100 (wet/dry) and no send.
 
If you get a difference in volume, it's down to adding extra signal paths and the gain controls in each path (send level, track fader level, bus fader level,  internal gain in the reverb wet/dry mix)

Simple rule of thumb, 
if you want reverb as a special effect on a single instrument (gated reverbed Phil Collins drum) then it goes in the FX bin.
If you need reverb for general ambience for multiple (or all) tracks, it goes in a bus.

ULTRABRA
So why the difference in result with the reverb as a send, compared to an insert?

Put another way:
 
With a reverb on a post-fader send, the the maximum wetness you can get is 50% because the dry signal is going to the Master bus from the output of the track at the same level as the reverb is returning to it via the reverb bus.(assuming the reverb doesn't alter the signal level). Only if you make the send pre-fader and set the output of the track to -INF or mute it are you going to get 100% wet signal with a send FX. That's why you typically run compressors, limiters and other dynamic FX as inserts - because you want only the processed signal.

^^^^
 
You'd typically have to set the reverb plugin's internal mix value differently when used as an insert in the FX Bin vs. a send (bus) in order for the results to match.