MS SSRS - Creating and copying custom palette

There are some preset palettes in Report Designer to use in charts. I did custom palettes because the variety of colours is not enough in preset palettes. I needed more than 10-13 different colours to the charts. I designed the custom palette using a colour pair system. In that system dark and light shade of same colour make one pair. That way bars in the chart or slices in the pie would be easier to recognize.

Errors while loading the designer - fixed in code view

During these months when I have build reports with Reporting Services, I haven't got many error messages but here is one that I met today...

I got following error message when I tried to open report file in Reporting Services designer:

One or more errors encountered while loading the designer.--- The errors are listed below. --- Deserialization failed: The 'DataType' attribute is not declared. Line 4921, position 20.

I could open report file in code view by right-clicking report and choseing "view code". I scrolled to that part of the code which was metioned in error message; line 4921, position 20. Error was in prompt parametervalue command/rule: DataType= "Integer".
Line was: <Value DataType= "Integer">0</Value> 
I removed that text ' DataType="Integer" --> <Value>0</Value>

After this correction report could be open in design view.