I believe the reason that MS Word is having a problem here is that it is interpreting the string:
- them using different colors
... as an antecedent noun phrase, them, modified by a so-called reduced relative clause, using different colors, where the whole phrase would mean:
- them [who are] using different colours.
Because we cannot usually have an accusative case antecedent for a relative clause, this would be ungrammatical. The word those, however, is not accusative case and is therefore a grammatical choice for such a construction:
- those [who are] using different colours.
As a noun phrase with a post-head modifier the example above is perfectly well-formed.
However, in the Original Posters example, using different colours is not modifying the pronoun them. Rather it is a gerund-participle phrase functioning as Adjunct. It is giving us more information about how the drawing was done.
In the example, them is functioning as the Direct Object of the verb draw. It is perfectly acceptable and grammatical. Many would argue that it is actually preferable to those here. The reason for this is that it is obvious that them refers to the domain walls. The reader needs no extra help identifying what is being picked out as the referent of the pronoun. Sometimes we might use those to express some attitudinal 'distance' from the thing being referred to. This is not the case here, so them might be the better choice of word.
Thoseis the plural form ofthat. It could be acceptable here. But I highly doubtthemis wrong. It is perfectly grammatical to use it after a verb to refer to something previously mentioned. – Tushar Raj May 09 '15 at 09:15