Problems with filtering by font color

[Regina Henschel]

Hi all,

LibreOffice has got the new feature in 7.3 to filter by font color 
(strings and numbers) in Calc.
Such color can be determined by
a) direct style assignment, including direct formatting
b) by number-format, e.g. red for negative numbers
c) by conditional formatting

Filtering is not just a layout operation, but effects the values in the 
spreadsheet via the function SUBTOTAL. Therefore, it is important to 
define the filtering precisely so that all applications produce the same 
values.

Unfortunately currently the behavior is:
LibreOffice: supports a)
Excel: supports a) and c)
Google spreadsheet: supports a), b) and c)
Gnumeric: not yet implemented, but wants to support a), b) and c).

How do you think filtering by font color should be specified in the ODF 
standard?

Kind regards
Regina