A screen cannot show you a wall
Your screen emits light. A wall reflects it. Those are physically different things, so no visualiser anywhere, this one included, can promise you the colour you are going to get.
On top of that, a colour changes through the day. Morning light is cooler, afternoon light is warmer, and a lamp at night changes it again. A north-facing wall and a south-facing wall in the same room, in the same paint, are not the same colour to look at.
What it is good for is ruling things out fast. It will tell you that the deep teal is too much for that room, or that two of your three shortlisted neutrals are indistinguishable and you can stop agonising. That is a real job and it saves a lot of tester pots.
How to test properly once you have narrowed it down
Test on at least two walls, because of the direction thing above. Judging a colour on one wall and rolling it onto four is how people end up repainting.
Paint a decent sized patch, not a smear. A small patch reads as darker than it will be once it covers a whole wall, and a patch surrounded by an existing colour is being judged against that colour rather than on its own.
Better still, paint the patch on a sheet of lining paper and move it around the room. That lets you see it against the woodwork, next to the window, and in the corner that never gets any light, which is usually the one that decides it.
The finish changes the colour
This catches people who have carefully tested a colour in the wrong sheen. The same colour in matt, eggshell and gloss reads as three slightly different colours, because the sheen changes how much light bounces straight back at you rather than scattering.
Matt looks deeper and flatter. Gloss looks lighter and busier, and it shows every imperfection in the wall behind it. Test in the finish you will be using, on the surface you will be using it on.
Why there are no brand codes here
These colours have plain descriptive names rather than references from one of the big paint or colour systems, and that is a deliberate choice.
Accurate values for those systems are a product their owners sell. The free charts of them circulating online are approximations of unknown provenance which disagree with each other. A swatch labelled with a code that is visibly wrong is worse than no code at all, because somebody orders paint off it.
When you get to ordering, the merchant's own fan deck under the light in your own room is the reference. Nothing on a screen is.
What to do with the area figure
Once the corners are on the real wall, the tool gives you the area of it. That is the number the paint calculator wants, so carry it across instead of measuring the room twice.
Two things to remember when you get there. Paint coverage on the tin assumes a sealed, even surface, and fresh plaster or a strong colour change will take more. And the openings matter: a room with a door and a window has meaningfully less wall than its dimensions suggest, which the room area tool works out properly.
The photo stays here
It is opened in your browser and never uploaded. No account, nothing stored, nothing sent anywhere. That is true across this site, and it bears saying on a tool where the first instinct is to wonder what happens to a photograph of the inside of your house.
Common questions
Will the colour look like this on my wall?
Not exactly, and no screen visualiser can promise otherwise. Your screen emits light and a wall reflects it, so they are physically different things. Worse, a colour changes with the light in the room through the day and with what is next to it. Use this to rule colours out quickly, then get tester pots.
Why do I have to measure the wall?
To place the fill on the real wall rather than on a rough rectangle, and so the corners land where the wall actually is. It also gives you the wall area, which is what you need for the paint calculator once you have picked a colour.
Where should I put a tester patch?
On at least two walls, because the same colour looks different depending on which way each wall faces. Paint a decent sized patch rather than a smear, and if you can, paint it on lining paper and move it around the room rather than putting four patches on one wall and judging them against each other.
Why are there no brand colour names or codes?
Because accurate values for the branded systems are a product their owners sell, and every free chart of them online is an approximation of unknown provenance. A swatch labelled with a code that is visibly wrong is worse than no code, because people order paint off it. These are our own named colours.
Does the finish change the colour?
Noticeably. The same colour in matt, eggshell and gloss reads as three slightly different colours, because the sheen changes how much light bounces straight back at you. Matt looks deeper and flatter, gloss looks lighter and busier. Test in the finish you will be using.
Is the photo uploaded anywhere?
No, and it never leaves the device. That matters here because the photographs people try are of the inside of their own home, usually with the contents of the room and sometimes the family in them, which is not something to hand over to find out whether a colour works. The picture is opened, recoloured and saved by your own browser, so there is no upload wait, no queue and no size limit beyond what your machine can hold.