Since signal power (or rms voltage) and noise power (or rms noise voltage) are, by definition, positive-definite quantities, their ratio can never be negative.
When the signal power is less than the noise power, then the signal/noise ratio is a positive value, less than one.
When translated into dB, a ratio <1 is a negative dB value.
In mathematical notation, a ratio can be written as A/B or A:B, which have identical meanings.
An archaic mathematical notation is A:B::C:D, which means A/B = C/D.
In English, that can be read as "A to B" or "the ratio of A to B".