Buying foundation online is one of those beauty rituals that can go brilliantly or terribly wrong. Get it right and you have a shade you can reorder for years; get it wrong and you are left with an expensive bottle of the wrong colour and a receipt you cannot bear to look at. The good news is that a little detective work before you tap "add to basket" removes most of the guesswork.

Start With Your Undertone, Not Your Depth

Depth — how light or dark you are — is the easy part. Undertone is where most online orders fall apart, because a shade can look perfectly matched on a screen and still leave you looking grey, sallow or flushed across the jaw.

Undertone generally falls into three broad camps, with olive sitting alongside them:

• Cool: pink, red or bluish undertones. The veins on your wrist tend to look blue or purple, and silver jewellery flatters your skin.
• Warm: yellow, peachy or golden undertones. Veins lean green, and gold jewellery tends to make your skin sing.
• Neutral: a balanced mix, usually with veins that are hard to define as either.
• Olive: a greenish or slightly grey cast that is often miscategorised as warm or cool. Olive skin commonly needs a shade labelled neutral so it does not read orange or ashy.

If you are still unsure, check the inside of your wrist and the skin just below your collarbone in daylight rather than under a bathroom bulb. If both silver and gold suit you, assume neutral.

Match Depth in Daylight, Never Indoors

Indoor lighting lies to you. Yellow bulbs make cool shades look pink, and fluorescent office light can add a green cast that vanishes outdoors. If you can, take a photo of your bare face and neck in soft, indirect daylight — a north-facing window is ideal — and keep it on your phone for comparison while you shop. Avoid direct sunshine, which blows out the midtones, and avoid camera flash entirely.

Then decide whether you are fair, light, medium, tan or deep, and take note of the depth of your neck, which is usually a touch lighter than your face. Foundation should bridge the two, not match the most tanned part of your complexion.

Read Online Swatches With a Critical Eye

Swatch photographs are your best available evidence, but they are rarely colour-accurate. Retailer images are often shot under studio lighting, and phone screens are calibrated for brightness and punch rather than truth. Look for swatches taken on real skin with a range of undertones, ideally on more than one model, and compare them side by side with a similar shade from another range.

• Check the swatch on the jawline and neck, not the hand or arm.
• Look for swatches photographed in daylight, or ask whether the image was edited.
• Note the shade family names — ivory, beige, sand, honey — but treat them as loose guides, since they vary wildly between ranges.
• Read the reviews that mention oxidisation, because a shade that turns orange after 20 minutes will not look the same on your skin as it does in the picture.

Order Samples Before You Commit

This is the single most useful habit you can build. Many brands sell sample sachets, mini bottles or a shade-matching kit for a few pounds, and some counters will decant a small pot if you ask nicely. Order two or three close shades rather than one — the difference of half a shade is often what makes the match invisible.

Read the returns policy before ordering, too. In the UK, unopened cosmetics can usually be returned within a set window, but opened foundation often cannot, so samples protect you where the policy will not. If you are shopping in person first, ask for a sample of the shade you like so you can wear it for a full day before buying the full size.

Test Properly When Your Parcel Arrives

Wait for daylight, then apply a stripe of each shade along your jaw and blend it down onto your neck. Leave it for at least 20 minutes, because many formulas darken or warm up as they settle on the skin. Check in a mirror by a window, then step outside — the neck is the honest judge.

• Choose the shade that disappears, not the one that brightens or warms you most.
• If you sit between two, go half a shade lighter when the formula oxidises, and half a shade warmer if your skin is olive.
• Consider finish and coverage, too: a matte formula can look a shade darker on dry skin, while a dewy one reflects light and reads slightly lighter.
• Buy the full size only once the sample has survived a full day, including a few hours of wear.

Finally, remember that your match can shift with the seasons. Many of us are a shade deeper in late summer and lighter by February, so keep a note of the shade you settled on and the reason it worked. Reorder with confidence rather than starting the whole experiment again, and if your skin changes — new medication, a holiday, a course of retinol — revisit your sample stash before you splurge on a bottle.