Which colours and numbers can you see in the photographs?
Ver good !
Absolutely correct. To be more descriptive, you could also say:
A red number seven on a green background.
A red fifty on a white background.
A black number ninety on a blue background.
You got that 100% correct -awesome !
A native speaker might say:
I can see the letter seven, on the green background.
The number fifty, is against the white background.
Ninety, is the number over the blue background.
These are three example sentences for you !