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.
A sheer fabric is mostly holes: fine yarns in an open weave block part of the light and let the rest straight through. What you see is two colours added together — the light the yarns reflect in the fabric's own dye, and the light that passes through the gaps from whatever lies behind, filtered slightly by the yarns it grazes. Over pale skin or a white lining the gaps read light and the colour looks pastel; over a dark layer the same chiffon looks deep and smoky. Layering two sheers multiplies the filtering and can produce a colour neither has alone.
Wear and care: Snagged or distorted yarns open the weave in patches, letting more of the underlayer through; synthetic sheers can also yellow slightly over time.