If you want to negate one constituent in an affirmative Turkish sentence how do you do it? I mean expressions like

         It was not me who chose this colour.

         It´s not about having enough time.

         This is not the way we deal with customers.

English sentences are very flexible. You can pick up just about anything and deny it without complex structures. I find these difficult to express in Turkish.

 Bu rengi seçen ben değildim.

Turkish have more possibilities where English has "not".

We have değil.

We have -me- negative sufix for verbs.

We have yok.

So going through your examples:

It was not me who chose this colour.

Bu rengi seçen ben değildim.

Bu rengi ben seçmedim. (We have a way of emphasing anything by placing it just before verb and stressing it with peaking intonation. Remember we had some discussion about emphasing recently. And see below for another example of it.)

Bu rengi seçen yoktu.

It´s not about having enough time.

Mesele (konu) yeterince zamanı olmak değil.

This is not the way we deal with customers.

Müşterilerle ilgilenme şeklimiz bu değil.

Müşterilerle bu şekilde/böyle ilgilenmeyiz.

More about emphasing:

Berke dün uçakla İzmir´e gitti. = Berke went to İzmir by flying yesterday.

Now we can change the order and using intonation we can give it different meanings:

Dün uçakla İzmir´e Berke gitti. = It was Berke who went to İzmir by flying yesterday.

Berke dün uçakla İzmir´e gitti. = It was İzmir Berke went by flying yesterday to.

Berke dün İzmir´e uçakla gitti. = It was by way of flying Berke went to İzmir yesterday.

Berke uçakla İzmir´e dün gitti. = It was yesterday Berke went to izmir by flying.

Funny. I was going to mix this question with emphasis (because negating a single constituent is a part of it) but it´s good I didn´t because you brougt it up in a better way. I have seen this talk about word order and emphasis many times but connecting it with negation brings it into a new light. Is it that when you organize the sentence in a way that puts a special stress on a certain constituent (placing it in the earmarked place before the verb) and then negate the predicate with -ma- you actually deny not the whole sentence but the stressed constituent? Or at least is this a possible interpretation?

         Berke dün uçakla İzmir´e gitti > gitmedi. (Is it ´yes, he travelled but not to Izmir´?)

Sorry, I didn´t understand the yok-example.

Hey Abla, did you mean you didn´t understand  about the ex "Bu rengi seçen yoktu" ? At least İ know "yoksa" is "if there isn´t", but I´ve not yet been able to get what -tu after yok is in Turkish. 

Right. It´s very indirect.

Bu rengi seçen yok = There´s nobody who chooses this colour (including especialy me).

And adding -tu makes it past tense.

And when it comes to querying a constituent Turkish is really superior. We have "mi" question particle and use it for asking about any constituent and without any touching the word order.

Bu rengi ben seçmedim = I didn´t choose this colour

Bu rengi mi ben seçmedim? = Is it this colour that I didn´t choose?

Bu rengi ben mi seçmedim? = Is it me who didn´t choose this colour?

Does değil change place as easily in the sentence as mı?

         Mavi değil bir halı satın aldım. (but the red one)

         Mavi bir halı değil satın aldım. (but the blue curtains)

Are these correct?

No they aren´t. But this way they would:

Mavi değil kırmızı bir halı satın aldım. (but the red one)

Mavi bir halı değil perde satın aldım. (but the blue curtains)

x değil y = not x but y

So you have to give the alternative. See, you have to explain it word by word even though it is no brainer to natives.