Use the tool
Type the month and day. The form posts to the finder's plain GET path, so it works with JavaScript off and lands on the date's page with its full palette.
The same question, one letter apart
In British, Irish, Australian, New Zealand, Canadian and South African English the word is usually written colour. In American English it is color. Both spellings name the same property of light, and both appear in searches for a birthday shade: the queries look like "birthday colour", "birthday colour chart" and "what is my birthday colour" as often as they look like their American counterparts.
Search engines have handled common spelling variants for years, so a page written in one spelling can still answer a query typed in the other. That tolerance is not a reason to ignore the difference. A page that never prints the spelling its reader uses reads as foreign, and a heading is the cheapest place to prove you are answering the question that was asked.
Why there is no separate British colour system
A birthday color on this site is one of 366 fixed entries: one for every date of a leap year, including 29 February. The entry is authored data, not a calculation from the visitor's browser, region or language. Switching the spelling in the URL does not switch the table, because there is only one table.
That is worth saying plainly, because "birthday colour" searches sometimes land on pages that imply a different tradition - a UK birth-color chart, a regional palette, a scoring quiz. No standards body publishes a month-color or date-color list, in either spelling. What exists is a family of designed systems, each with its own mapping. This one maps dates; the spelling you used to find it does not move your date.
What the answer contains
Enter a month and a day and the finder opens that date's page. The page names the color, prints its hex value, shows RGB and HSL values, gives a single color word and a short reading, then builds the palettes around the color: complementary, split-complementary, triadic, analogous, tints and shades. The same page measures four text-and-background pairings and prints their WCAG 2.1 contrast ratios.
January 1 is January Ironwork, a cool slate blue, with the word CALM. September 18 is Pine Conscience, a green, with the word BRIGHT. Those values are the same whether a reader arrived by typing colour or color, and they are stable from year to year: the date is the input, and the calendar does not consult the year. The exact hex values are printed on each date's page and on the tool below.
The colour word is not a translation
Under each color name sits a word in capitals - CALM, TENDER, BURNING, OBSCURE. It is not a translation of the color name and not a second name for the hex. It is a one-word editorial label chosen with the reading, and it stays with the date in every language; the translation changes the language, not the claim.
The reading below it is written for entertainment. It does not predict health, money, love or temperament, and the site says so on the terms page, the editorial policy and every day page's own copy. A spelling preference is not evidence about a person, and neither is a color.
Which spelling this site uses, and where
The product name, the URL paths and the English interface use the American spelling: Birthday Color Finder, /birthday-color-chart, /what-is-my-birthday-color. The editorial and engineering notes behind the site sometimes use colour, because the people writing them do. That is an inconsistency in the workshop, not in the product, and the public pages keep one spelling per language.
If you are writing about the site, either spelling is fine in your own prose. If you are linking to it, the paths are the American ones; the redirect from a misspelled path is not something to rely on, because there is no such redirect. Link the canonical URL the page declares and the spelling question stops mattering.
How to use either answer
Copy the hex value if you are matching the color elsewhere. Use the palette if you are choosing a second color: the harmony angles are computed from the hue, so the advice travels with the date rather than with a season. Use the contrast table before putting text on the color, and use the printable chart if you want the whole year on paper.
The one thing not to do is treat the reading as a test result. There is no score, no ranking and no correct answer to compare with a friend. Two dates produce two colors; the interesting part is what you do with the hex.
Where the two spellings came from
Samuel Johnson's Dictionary of the English Language, published in 1755, used the -our spellings that were already common in English: colour, honour, favour. Noah Webster's American Dictionary of the English Language, published in 1828, used -or for many of the same words and argued that spelling should follow pronunciation and be simpler. American dictionaries largely settled on color; British, Irish, Australian, New Zealand, South African and Canadian usage kept colour. The split is a dictionary and publishing history, not a difference in what the word means.
Canadian English is a useful example of how mixed the rule is: it keeps colour and honour, while often using American spellings and vocabulary elsewhere. A reader in Toronto is not confused by color, and a reader in London is not confused by colour. The variant is a convention, not a boundary. That is why the site answers both and prints the one each page uses, rather than pretending the other spelling is a mistake.
The practical consequence for a URL is small. Search engines treat common spelling variants as related queries, and a canonical URL is one address rather than a claim about spelling. What matters for a reader is that the page they land on uses their spelling in the heading and then answers the question. The palette beneath it is the same 366 entries either way.
Questions
Is "birthday colour" the correct spelling in the UK?
Yes - colour is the standard British spelling, and color is the standard American one. Both describe the same lookup on this site, and the date's color is identical either way.
Does my spelling change the color I get?
No. The palette is one fixed table of 366 dates. The spelling you typed changes nothing about the date, the hex value or the reading.
Why do the URLs use the American spelling?
The product name and the English paths were written with the American spelling from the start, and those paths are the ones already linked and indexed. Changing them would break links without improving an answer.