Optional multi-valued parameters in SSRS

It sounds like you undertand your situation well: You cannot pass the value of NULL as a parameter, because NULL simply is not a value and has no value. You could pass the string "NULL" as the parameter @color, but you'd probably be better off creating a colorfully-named (shall we say, distinctive?) distinctive variable, such as noColor just to keep things clear.

What you can still squeeze comfortably out of SSRS:

• A multi-value company parameter based on a dataset;
• A cascaded (from company) multi-value department parameter based on its own dataset;
• An optional multi-value employee id parameter, based on a dataset that might filter on company/department;
• An optional custom employee id parameter, plain INT input;

Your datasets would be something as follows.

For @Company:

SELECT CompanyId,       -- Param value
       CompanyName      -- Param display in SSRS
FROM   vw_AllCompanies

And for @Department:

SELECT DepartmentId,    -- Param value
       DepartmentName,  -- Param display in SSRS
FROM   vw_AllDepartments
WHERE  CompanyId = @CompanyId

And for @EmployeeId:

SELECT EmployeeId,
       FullName
FROM   vw_Employees
WHERE  (DepartmentId = @DepartmentId AND CompanyId = @CompanyId)
       OR (@DepartmentId IS NULL AND CompanyId = @CompanyId) -- Optional

Then your main dataset would do:

SELECT * -- Okay, don't use "*", but select actual columns :)
FROM   vw_AllMyData
WHERE  EmployeeId IN (@EmployeeId)            -- The cascaded param
       OR EmployeeId = @SomeCustomEmployeeId  -- The custom INT input param

In my experience, this is slightly clunky, but probably the best you can get out of basic SSRS. If you want more flexibility I recommend you built something in your app around it, and pass the ID as a final parameter to the report.