Guys, I am a 50 y/o man working in the top rungs of a Petroleum
Marketing company. I am 171 cm tall and weigh 75 Kgs. I am a
fair-looking Indian. I wish to look elegant and sell my self with good
appearance besides good performance. What kind of dress should I wear
? What suit material ? What shirt material ? What tie material ? What
colours ? Are the ready-mades better than tailor-mades ?

Don't wear a dress !!!!!! 

Its just all wrong for a 50 yr old business man ;)

Steph53

Well from your description I would say that a business casual style
would work very well for you.  For normal every day wear I would say
medium to dark tan, grey, and black dress pants, or even a dark green
pair of dress pants.  With the cotton style dress shirts, make sure
the colors compliment with the pants, like black shirt with tan pants
or mainly white to light tan with black pants.  Shoes should be a
dress style but not the really expensive ones or the fancy ones, stick
with a dress style but one that looks like it could be walked in for
many miles without showing too much wear.  For days wear you want to
really dress to impress I would recomend a suit, it sounds like you
would be able to wear a three button suit, don't use a vest and I
would go with a tailor made one.  For colors I think a dark grey would
compliment your complexion or even a medium navy blue, and of course
black, but a black suit makes anyone look good.  For casual outside
wear I think getting a few pairs of cargo pants that are not jeans
would look good and you could pick up a few button down shirts that
are not the really nice style but still dressy, then pick up some
plain black, grey, and white undershirts and wear the button shirt
unbuttoned and untucked but tuck in the undershirt.  Well that is my
advice, I hope it helps.

Greetings,
As you have already discovered here, you can get quite varied responses.

It seems that you have already sold yourself through good performance,
congratulations!  That also suggests that you have probably been
dressing quite correctly all along, but maybe you have been very
consciously adhering to the US/UK (?) "dress code", which is probably
the best way to go if one is recognizable as foreigner.
 
I would say, if you are staying in the same company, or if you been
newly hired, either way, it would be found surprising if you suddenly
changed your style.  My first thought is that the gossip in the old
company would be that you must have a new lady friend; and in the new
company (if that were the case) that you had been "selling yourself"
in the interviews - and been successful - but now the "real you" was
coming out, somewhat in contrast with the image of the person the co.
thought it had hired.

If you want to enjoy being able to dress better now, I would recommend
better quality rather than a new look;  yes, a tailor-made suit, but
in more or less the same style that you have been wearing.  Perfect
tailoring makes anyone look better  - and feel better, not just for
knowing, but because you feel that a tailored jacket fits perfectly. 
Others will notice that you are well-dressed, but maybe not recognized
why, but that isn't important if they have the impression that you
look good.
The same for shirts, maybe now all with French cuffs and small gold
cufflinks, even with a sportcoat.

Someone once said that a gentleman's clothing should never call
attention to itself (maybe that was Lord Chesterfield in his letters
to his son - a bit far back - but still pertinent).  And someone else
said that you can recognize a gentleman by his shoes: conservative
quality, even they are obviously old, but well kept and he is wearing
tatter jeans.

OH, and I bet you have at least one outfit in which you feel especial
well dressed, that maybe has elidited compliments.  That is your
style, build on it.

Best regards

Any good men's clothier should be able to help you with that.  Be sure
to keep extra clothes in your office, if possible.  You never know
when you might spill ink or food on yourself right before a big
meeting.  Keep an extra tie and shirt, at the very least.

I think in most cases shoes are easily ovelooked by most guys.  I got
leather oxfords at http://www.0-day.biz that I am really happy with. 
Also try Zappos at http://www.zappos.com .