Short answer: a rainbow on a screen protector is light reflecting off two surfaces very close together. Most of the time that means a thin layer of air under the glass. Sometimes it is polarised sunglasses, and sometimes it is the texture of a matte sheet. Only the air is a fault, and it is the one you can fix.
The ten-second test
Press gently on the rainbow with a fingertip. If it moves, shrinks or changes colour, there is air under the glass. If nothing changes, take off your sunglasses or tilt the phone: if the pattern disappears, it is polarisation, not the protector. If it is a fine, sparkly shimmer across the whole screen and your protector is matte, it is the matte texture.
Air under the glass — the fixable one
When the glass is not bonded flat, a gap a few thousandths of a millimetre deep opens between it and the screen. Light bouncing off the top and bottom of that gap interferes with itself and splits into colours — the same physics as a film of oil on a wet road. It shows up as rainbow rings or patches that drift when you press.
There are two usual causes. A speck of dust holds the glass up around itself, and you will see the particle in the middle of the rings. Or the sheet only has adhesive around its edge, and the middle floats. Cheap edge-glued sheets often show a rainbow patch in the centre for exactly this reason.
For dust: lift the glass from the nearest corner with a strip of tape, touch the speck with a dust sticker, and lower it again. For an edge-glued sheet there is no fix — the middle was never meant to stick.
Full-surface adhesive is what prevents this. It bonds the whole sheet to the panel, so there is no gap anywhere for light to split in. It is also what keeps an in-display fingerprint reader working, which is why it is worth checking for before you buy.
A rainbow line along the edges
On a phone whose screen curves away at the sides, a thin rainbow line along the edge means the glass has lifted where the screen bends. Rigid glass cannot follow a curve, so a sheet cut to wrap the bend sits on an air wedge at the edge — and that wedge shows colour.
The fix is a sheet cut to stop just inside the curve, so the whole of it lies flat and bonded, at the cost of a slim border. UV glass, bonded with a liquid adhesive cured under a lamp, is the other route on curved screens. Pressing the edge down does not help: it springs back.
Polarised sunglasses — not a fault
Tempering is fast, controlled cooling, and it leaves the glass with built-in stress. That stress is invisible in normal light, but polarised light shows it as rainbow or mottled patches. Phone screens give off polarised light, and polarised sunglasses filter it, so the combination can show a pattern that is simply not there to the naked eye.
Nearly all tempered glass does this to some degree. It does not affect the protection, and it changes as you rotate the phone. If it bothers you outdoors, the answer is the sunglasses, not a different protector.
Sparkle on a matte protector
A matte sheet is etched or coated to scatter reflections. On a sharp, high-resolution screen, that texture can split light into a faint coloured sparkle, most visible on white backgrounds. It is the price of the anti-glare surface. If you want none of it, a clear sheet is the one to choose.
If you were searching for a rainbow-pattern guard
Some listings called "rainbow screen guards" are decorative back films with a colourful, shifting pattern. That is a different product: a film for the back of the phone, chosen for looks. Everything above is about rainbow patterns you did not ask for, on the front.
Questions people ask
Why does my tempered glass show a rainbow effect?
Usually because there is a thin layer of air between the glass and the screen — from a speck of dust, an edge-only adhesive, or a sheet lifting at a curved edge. If the pattern only appears through polarised sunglasses, it is stress in the tempered glass showing up in polarised light, which is normal.
Is the rainbow effect bad for my phone screen?
No. It is an optical effect, not damage. Air under the glass can let dust in and an in-display fingerprint reader may struggle, which is why it is worth fixing, but the screen itself is not harmed.
Will the rainbow on my screen protector go away on its own?
Not if it comes from dust or an edge-only adhesive — those need lifting and cleaning, or a better sheet. Tiny clear patches near the edge in the first day can be adhesive still settling, and those usually clear within 24 hours.
Why do I only see a rainbow with sunglasses on?
Phone screens give off polarised light, and tempered glass carries stress from the tempering process that polarised lenses make visible as coloured patches. Take the sunglasses off or rotate the phone and it changes or disappears.