I'm a Parker 45 collector (among other pens). My impression is that the "Happy Colors" Yellow is quite hard to find. I have two FPs of one type of yellow, and one of another type of yellow. See the photos here from different distances and perspectives:
Based mostly on the photo of the barrel ends, I believe that the two yellows (what I'd call a "goldenrod" yellow) that are alike are the authentic Parkers, and the one yellow that differs from the other two (what I'd call a "bright" yellow) probably is an Ariel Kullock creation. The bright yellow barrel end has a different shape from other authentic Parker 45 FPs and seems to match other Kullocks of the same pen model. The other reason I believe that the bright yellow 45 is a Kullock is that one cannot screw off the nib section (the pen was never used before I purchased it, but I soaked it anyhow); the nib section appears to be fixed by design.
On the two goldenrod yellow 45s, I replaced the stainless steel fittings, cap, and nibs with gold-plated fittings, gold arrow and brushed steel caps, and gold nibs (because I prefer the gold as a contrast to the stainless steel). So don't be misled by my changes.
My question to the group is whether my conclusion about which yellow version is an authentic Parker is correct or incorrect. Your input would be appreciated. Thanks. -- Donn