How to produce an output of fields that are NULL (combined from multiple columns+tables)?
Try this:

WITH CTE(Assessor,Date,Field) AS (
select Assessor,Date,Field
from(
select Assessor,Date,ISNULL(Length,'Length') as Length,ISNULL(Colour,'Colour') as Colour,ISNULL(Weight,'Weight') as Weigth
from TestTable) as OP
unpivot(
Field for Fields IN (Length,Colour,Weigth)) as UP)
select Assessor,Date,Field from CTE
where Field in ('Colour','Length','Weigth')