1) I think, there is a general preference for ং at the end of words. So, রং 'colour' is definitely more common than রঙ, which looks almost wrong to me (pun intended). 🙃
2) It is impossible to have a vowel directly follow a ং in the same word. So, we have রং 'colour', but usually রঙের 'of a colour'. Sometimes, you see an attempt to preserve the ং in the second instance by writing রং-য়ের or রং-এর, but I think it is just unnecessarily complicated.
3) Inside a word, the conjunct letters ঙ্ক, ঙ্খ, ঙ্গ, ঙ্ঘ seem to be preferred to ংক, ংখ, ংগ and ংঘ in Sanskrit words, even though both spellings are correct in Sanskrit itself. So, there is a little bit of variation, e.g. the word অঙ্ক can mean (a) Mathematics/school mathematics problem, or (b) lap. The first meaning is widely used colloquially and is as (if not more) likely to be spelt অংক as অঙ্ক, presumably because it is so common and does not feel very Sanskrity. So, the spelling with ঙ্ক is not really preferred. But the second meaning is always spelt অঙ্ক, because it is a high register use, which feels very Sanskrity.
4) For other sequences than those in (3), only the spelling with ং is possible, e.g. সংবাদ 'news', সংযত 'restrained', etc.
(4) and (2) should also explain why we write বাংলা 'Bengali, Bengali language', but বাঙালি 'Bengali person'.
PS: A little bit more can be said about the consonant sequences in (3) and (4) from the Sanskrit perspective, but they are not necessary in a Bengali context.
