Hi Kumar,
When a tree cell shows the text, what is does is it takes the Value of that cell and calls the ToString method on it and passes in the Format you specified.
So the formats that are supported for any column depend on the data type of the value of that column. It sounds like you are using Strings here, and the String data type does not support any formatting. Formatting is only supported for numeric types.
So you have a couple of options.
1) You could change the Value of the cell to display the text you want.
2) You could use a DataFilter to add / remove the colon.
3) You could use a DrawFilter to draw the text in the cell yourself. This method is really only useful if the field is read-only.
I'm searching for a similar solution and it would be a very powerful feature if the Format property could take a string with {0} as wild card; equal to string.Format(format, value).
e.g
Value = 8.152735fFormat = "{0:c}"
Displayed as 8,15 €