Hi,
Interesting question, Les.
I know that my vocal tone differs greatly, depending on the material, the style, the venue, the audience, and whether I'm solo, or with others.
I also believe that it's a good thing to sing in the voice that is one's "real" voice. While it is only natural to want to sound like your musical heroes, over time, and with enough listening, it is possible to find the true sound your vocal chords make.
That said, colouring your voice depending on the above-mentioned variables is only to be expected, and indeed, part of the fun.
I can't tell you the number of times I've felt frustration listening to friends singing and hearing the true tones fighting to get out, only to be squashed by their efforts to sound like Neil Young, or Jewel.
Find your own sound, and choose to use it.
Owl.