[jocko]

70 501 is a one-year only color, and note that it is a metallic paint. 71+ code 510 is also "medium blue" but it is non-metallic and a slightly different shade. 501 appeared one of 5 ways:

501=Solid Med Blue Metallic
531=521 White over 501 Med Blue Metallic "conventional" two tone.
591=521 White over 501 Med Blue Metallic over 521 White "Special Two Tone"
558=508 Dark Blue Metallic over 501 Medium Blue Metallic "Custom Two Tone"
559=inverse of 558

Also of note, the meaning of 558/559 changed in 71 - although the exact same Custom Two Tone RPOs remained, they became the nonmetallic versions of the similar combo, i.e. in 71-72, 558=523 Dark Blue over 510 Med Blue (and 559 was the inverse) - all non-metallic.