If you used this prior to October 12th 2016 then you need to update your existing bookmark.
I've written a little application to help you edit your crew colour. You can either enter a hex value, use the colour picker, or load an image from a URL and pick a colour directly from it. Rather than have some massive piece of code in a bookmark I've simplified the whole thing into one line. Simply copy and paste the following code...
Then create a new bookmark in your browser and call it something like "Edit crew colour", and paste the code you just copied as the address or URL.
When you go to your crew page and you're in the edit tab (where the crew colour can be set), just click the bookmark when you want to edit the colour and it will pop up the dialog as shown here...
Use that dialog to set your crew colour to anything you like, and then click "ok" to set it. You must also save your changes with the save button at the bottom of the page, as before. If it tells you that it couldn't save for some reason then it will tell you it was attempt 1 of 32 (currently - that may change). Click save again and it will try a different combination that may save your colour. It's rare that you'll have to do this more than a few times and unlikely you'll ever find a colour that won't save with any of the 32 tries.
If you want to change the colour again you can either click the bookmark or just click the colour button on the page. Once the script is loaded it will stay available until you leave the page.
To use an image to choose a colour, either type or paste the image URL into the text box in the dialog and press enter to load the image...
You can then click anywhere in the image to select the colour under the mouse. This is helpful for when you have photos of vehicles and you're trying to match the colour. It will never be exact as the colours in the game are slightly different from what you see in real life, but you can get close enough and then make minor adjustments from there, should you need to.
If you have any requests for help with finding the right colour then please don't ask for them here. There's already a very good thread where there are lots of people helping out with that...
If you have any problems with this utility then just let me know.
28th November, 2017
Upped the number of "recently used colours" to 24, like it should have been from the start! Many thanks to Randoom. for the heads-up about that.
15th June, 2016
Added a "recently used colours" box where any colours you recently saved are shown, up to a maximum of 10. (It only shows colours saved from now on.)
9th June, 2016
Fixed issue where the colour picker was picking the wrong colour if the browser page had been scrolled at all.
8th June, 2016
Updated website to use https protocol, in order to work with the recently updated Social Club.