it's a "rule of thumb" not an absolute

FY1 - 1970 series "lemon twist"
(D)Y2 - 1968 series "sunfire yellow" (paint code "S" during 1968)
GY3 - 1971 series "curious yellow"
FY4 - 1970 series "citron mist metallic"

1970 FY1.jpg1969 Y2 ROAD RUNNER.jpg1971 GY3 ROAD RUNNER.jpg1970 FY4 Road Runner.jpg