SSRS: Alternate colors for grouped rows issue

I came across a bit of a problem. I want to use alternate colors for rows grouped by department. I found three methods to realize this:

1. Using a piece of code determining if the row is even or not and based on that piece of code setting the background color. (http://olsv.blogspot.com/2009/08/alternating-background-colors-for-odd.html)

2. The second one is using the following expression in the background properties: =iif(RowNumber(Nothing) Mod 2, "Green", "White"). (http://msdn.microsoft.com/en-us/library/ms157328.aspx)

3. The last one is the same as the previous one, only the expression is different: =IIf( RunningValue (Fields!FieldName, CountDistinct, Nothing) MOD 2, Color1, Color2). (http://weblogs.sqlteam.com/joew/archive/2008/08/20/60691.aspx)

All methods work fine and the rows even and odd get alternate colors grouped by department....but… here is the problem: The report has an calculationfield. I sort my tablix on that field. The ordering is correct, but it colors get mixed up. They are set on the non-ordered set.

So I understand that SSRS first sets de background colors and then sorts the tablix, which leaves me with a bit of a problem.

I will post the fix when I find the solution.