I'd describe this colour as a peachy nude with a slight silvery shimmer that's only obvious when the lipstick is taken off. Shy Girl has a subtle but noticeable sheen. In warmer light, it appears to border on a light coral. It's a Cremesheen lipstick which I've always thought was meant to be more moisturising and creamy, but I find it a bit dry and hard in texture. However, from my experience, MAC lipsticks, like Revlon, are generally quite forgiving on the lips, in that they can easily be applied straight from the tube and don't tend to settle into fine lines. Shy Girl is one of those effortless everyday colours that you can reach for without thinking. It's dark enough to not wash the complexion out, but peachy enough to not be a traditional brown-leaning nude. It's difficult to find a neutral lipstick that is the right colour and easy to wear. Shy Girl fits the bill.
l-r: MAC Shy Girl, Revlon Charm, Revlon Soft Nude, Rimmel Nude Delight, Face of Australia Iced Almond
It turns out that I have a few similar shades, but no exact dupes. The closest to Shy Girl is probably Rimmel Moisture Renew lipstick in Nude Delight, which makes sense since Nude Delight is one of my favourite, though oft-neglected, nude lipsticks. Face of Australia Lip Quench in Iced Almond looks very similar to Nude Delight in the swatch, but on the lips it's actually more of a pale, slightly pinkish beige, with a very creamy formulation that I find difficult to work with. Revlon ColorBurst lipstick in Soft Nude is significantly lighter but still has that distinct peachy tone of Shy Girl. Revlon Just Bitten Kissable Balm Stain in Charm is the sheerest and palest, but it stains more coral, so somewhere between when it's first applied and when it stains the lips would be the closest approximation to Shy Girl. I've always loved the colour of Charm before it inexplicably changes colour, so finding Shy Girl means I can now happily wear that shade all the time.