These emoji characters are also useful if you are okay with this limited variety of colors and shapes (though they may look different in different OS/browsers), This is an alternative to AlecRust's answer which needs an external service that may go down someday, and with the idea of using emojis from Luke Hutchison's answer:
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There are also many colored rectangle characters with alphanumeric/arrow/other-symbols that may work for you.
Also, the following emojis are skin tone modifiers that have the skin colors inside this rectangular-ish shape. Don't use them! Because they should be alone ( otherwise they may modify the output of the sibling emojis) and also they are rendered so much different in different os/version/browser/version combination when used alone.