Incorrect hue gradient used

[Zyets]

I don't know how you get that values... for me it looks like you randomly picked them from some sample...
That are correct:
#ff0000 0%,
#ffff00 16.7%,
#00ff00 33.3%,
#00ffff 50%,
#0000ff 66.7%,
#ff00ff 83.3%,
#ff0000 100%

here is some math logic used, dude
hue wiki

ps i like your rgb2hex method though)) its awesome!
Best regards)

[simonh1000]

I had a go with @Zyets suggestion and it seem to work well except near the yellow