Just to be clear....Ch. 7 of the AMT Handbook is referencing identification of aircraft fluid lines. Extending that practice to filler caps is quite a leap.
What if it's a wet wing airplane? Or a Cherokee? Or a fiber/matrix composite?Some folks like to see the whole tank painted the appropriate color-code. Your mileage may vary.
There is nothing to prohibit a fuel cap from being red, or orange, yellow, green, blue, indigo, or violet so if you want your cap any of those colors, go for it. Expressing personal preference can be a good thing.