Phasing Order in IEC Color Codes
Is it common practice or is there some other standard for Europe only
  for example?


In Europe, it would be best to follow 60445:2010, but this is most likely similar to the old 60446, they moved the requirement and this was merely a documentation change (as far as I can tell). Also as far as I can tell, CENELEC has followed the IEC standard since 2004


  Three-phase cables
  
  For three-phase cables, rather than accept the European consensus –
  which is to colour code the phases brown or black – the UK committee
  considered alternatives. The importance of clearly identifying the
  individual phases using different core colours was acknowledged.
  Accordingly it was decided to propose alternatives for consideration
  by other European countries, rather than adopt the harmonized system.
  The committee proposed that three separate phase colours be used for
  flexible and fixed rigid cable cores. The colour of the
  earth/protective conductor was to remain green and yellow.
  
  The committee succeeded in persuading the other European countries to
  adopt separate phase colours (brown/grey/black) and in May 2001
  CENELEC HD 308 S2 was published. This has just been implemented in the
  UK, by BS 7671Amendment 2: 2004. Issued on 31 March 2004, this
  amendment also includes some editorial changes and references to the
  Electricity Safety, Quality and Continuity Regulations 2002.


Source: https://www.thenbs.com/knowledge/do-not-get-your-wires-crossed-the-colour-coding-of-low-voltage-fixed-wiring-cable-cores

The CENELEC standards now mirror IEC standards.

The old 2007 version of 60446 also listed the colors as this:


  Section 5.2.3 For AC-Phase conductors the preferred colours are BLACK,
  BROWN, and GREY


Source: https://webstore.iec.ch/p-preview/info_iec60446%7Bed4.0%7Den.pdf (now withdrawn)

The standard has been withdrawn; the fourth edition (IEC 60446:2007) was merged in 2010 into the fifth edition of IEC 60445 along with the fourth edition, IEC 60445:2006. 
Source: Wikipedia 60446

CENELEC follows the IEC codes. Not a big deal if you are in europe, if your in the UK then the colours changed drastically:


  In 1977, CENELEC published Harmonized Document 324 S1, covering the
  identification of insulated and bare conductors by colours. However
  the UK was not obliged to harmonize with this because, at this time,
  the IEE Wiring Regulations were not a British Standard. It was not
  until 1998, when CENELEC revised HD 308 S1 to include rigid cables
  within its scope, that the UK committee responsible for BS 7671
  Requirements for electrical installations decided to review the UK
  position. By now the UK was the only country within Europe not to use
  harmonized cable core colours. The committee decided that, for
  single-phase installations such as those in the home, core colours of
  fixed rigid cables should be changed to match those of flexible cables
  such as lighting pendants:

Live - Brown
Neutral - Blue
Earth/protective conductor - Green and yellow.

  
  This will remove the colour differences between fixed and flexible
  cable cores. The UK public are already familiar with the colour coding
  of flexible cables and it is anticipated that this change will not
  result in too many problems.


Source: https://www.thenbs.com/knowledge/do-not-get-your-wires-crossed-the-colour-coding-of-low-voltage-fixed-wiring-cable-cores


  So, what is going on here? Is it just common practice or is there
  some other standard for Europe only for example?


No, the confusion is in the transition from standards. In European countries the standards have been around for a while and follow the IEC/CANELEC standards.

The standards are determined by country, some have adopted NEC standards or IEC standards. There is a table listed here(and pictured below), that can show the relation between countries and standards. 


  When you are working on a project that is in a foreign country it can
  be difficult to know which electrical standards are applied to the
  building code – the National Electrical Code (NEC) or the
  International Electrical Code (IEC).  Most of the world, outside the
  U.S. and its neighbors, have adopted a version of the International
  Electrical Code. For example, all of the European Union countries use
  a version of the IEC, however, they have small differences and
  sometimes create whole new translations for their local languages.
  Some countries have created their own version of the International
  Electric and others have adopted editions from other nations. Needless
  to say, with multiple codes and multiple versions of electrical codes,
  it can be confusing which countries use exactly what code.


Source: http://www.esgroundingsolutions.com/nec-iec-electrical-standards-listed-country/

The standards determine the wire color, some countries deviate or add from the main standards. So it becomes imperative to look at the national electrical codes for the region you are working in. 

It is my thinking that the variations come from trying to maintain backwards compatibility in wiring colors because the IEC standards came after the national standards were in place.


Source: esgroundingsolutions