It is offensive to Jews of colour to describe all Jews as white
Jews, like many groups bound by their religious convictions, count millions of people of colour amongst their number. For centuries, large parts of Goa, Ethiopia, Egypt and the Middle East have had thriving non-white Jewish populations. Any argument that claims all Jews are white is therefore reductive, offensive and patently untrue.
It is offensive to Jews of colour to describe all Jews as white. This argument rests on the assumption that by stating all Jewish people are white, it is fundamentally exclusive to the millions of people of colour that identify with the Jewish religion. For thousands of years, Jewish people of a variety of races and ethnicities have practiced Judaism. Large parts of Goa, Ethiopia, Egypt, and the Middle East have large populations of non-white Jewish communities. Furthermore, in an increasingly diverse world, there are many families that feature interracial couples where one or more have converted to the Jewish religion. By declaring all Jewish people are white, it disenfranchises all of these individuals who thoroughly believe in their religion and lifestyle. Therefore, such a statement is discriminatory towards these communities. As Judaism is primarily defined as a religion, it therefore can count members of all races as part of its constituents. The concept of Jewishness being tied to whiteness is entirely exclusionary in nature, as well as untrue. 
Being Jewish, while certainly a religion, is also tied fundamentally to race as supported by DNA research. This same fact is untrue in comparison to other majorly popular religions. Therefore, to compare what is inclusive in Judaism and what is inclusive in, for example, Islam or Christianity, is null as they are discussing two fundamentally different root ideas. As Jewishness is tied to race, it is thus not exclusive to illegitimate Jews of colour, it is merely fact.