You need to know which convention is being used, because there are more than one.
One system simply uses upper case Roman numerals to indicate scale degrees with no reference to chord quality or scale type.
For systems that use sharps and flats the basic idea is they alter the referenced scale degree from some prevailing default.
In jazz and pop that default is a diatonic major scale. In the key of
C - assumed major - the sixth scale degree is
A natural and the diatonic triad built on it is a minor chord. The Roman numeral for it is:
vi. If you want to indicate the sixth degree and diatonic triad from minor, you use a flat to show the scale degree is lowered from the major scale degree to
A flat and use upper case to show the chord quality is major:
In 'classical' analysis it works differently. You indicate the key before writing the Roman numerals:
Cm:. Then the Roman numerals assume what is diatonic for that key signature. In
C minor the sixth degree and it's diatonic triad are
A flat and a major triad by default - because of the key signature - no alterations are made, so no accidentals are added to the Roman numerals.
Cm: VI means a major triad built on the sixth degree in
C minor which is
A flat. On the other hand, if the key was major:
C: and an
A flat major chord was played, it involves a chromatic alteration from the key signature which is applied to the Roman numeral:
C: bVI. That's called a borrowed chord.
A flat major was "borrowed" from
i, VII, VI, V
(because B♭ is the 7th step of the C minor scale, etc.)
I think if you write:
Cm: i, VII, VI, V, it will be perfectly clear you mean 'Cm, B♭, A♭, G'.
Without indicating the key -
i, VII, VI, V- could be interpreted as
Cm B A G. Realistically, people familiar with various systems will assume you meant
Cm: i VII VI V or
i, ♭VII, ♭VI, V. But why not write it clearly one way or the other?