I would not use that language. The T-shirt is black, blue, and orange.
Can Sub-color mean secondary color?
Primary color of T-shirt is black and the sub-colors are orange and blue.
Or 'The T-shirt is mainly black, with bits of orange and blue'.
Why? Is that language inappropriate?
So, a subcolor of a t-shirt doesn't mean it's a second color?
'Subcolour' is a self-coined word, not a accepted word when talking about colours.
Only the writer knows the meaning of the term. We can only guess.
Black is a neutral colour, not a primary colour. The primary colours are blue, red and green for lighting and blue, red and yellow for printing.
Orange is a secondary colour obtained from mixing red and yellow.
I am in the design field dealing with colours, so I know the terms.
not a teacher