A sheer fabric only partially blocks light, so what's layered underneath or behind it — skin, another garment, or a background — visibly mixes with the fabric's own colour, which is why the same sheer piece can look quite different over different layers.
Practical advice
Judge a sheer fabric's colour layered exactly as it will actually be worn, not in isolation against a plain background.
Always confirm with the physical garment under real light before committing to a colour choice — see methodology.