Add a calculated measure to your query, using the following expression:
[Measures].[Chicken Count] / ([Measures].[Chicken Count], [Chicken].[Colour].[All])
The tuple in the denominator removes the local filtering of chickens by colour (assuming the name of your dimension is called 'Chicken' and the attribute is called 'Colour'), returning the total count of chicken in the given context.
Similarly, if you wanted a farm percentage of total number of chickens, you would do this:
[Measures].[Chicken Count] / ([Measures].[Chicken Count], [Chicken].[Colour].[All], [Farm].[Farm].[All])