The "official" language for Moodle is actually the Australian English (hey mate!) and the default locale is set to en_AU.UTF8. That's basically the same as the British English though. So from this point of view, "Select your color" is a typo. However, some strings are copied over from upstream projects (like TinyMCE) which might be using en_US. So for core_editor and editor_tinymce strings, I would suggest to leave them as they are. They would be probably overwritten during the next upstream upgrade anyway.
There are some exceptions, too. Especially when the string eventually refers to an HTML or CSS syntax. Then the US spelling is correct (for example, some strings still use the legacy <font color="#ff0000"> tags or they refer to CSS property "background-color").
We do not fix typos in string identifiers usually. However, feel free to submit a patch for "color" in all other places - like themes (I used the AMOS Translator filter and found couple of them).