ID Sunbird in Oman - White, Dark Grey, Yellow
There are 4 species of Sunbirds in Oman. 
https://en.wikipedia.org/wiki/List_of_birds_of_Oman

Based on your description and how common are the different species I think they were most likely mainly Purple Sunbird (Cinnyris asiaticus) https://en.wikipedia.org/wiki/Purple_sunbird
At the moment, in fact, they are not purple at all. They are moulting their feathers and also the male have extensive yellow on breast and belly. Among the species of Oman's Sunbirds it is the only one that shows black wings (in males) contrasting with the brownish/grey upper parts precisely as you observed.

MOULTING MALE


FEMALE (left)