ATSF "yellow"

[Olivani]

"It's sun-faded!"

That's exactly it. Just look at prototype pictures. Sure, the way the picture was taken has some influence as well but there is rarely a locomotive with the same shade of yellow, red or orange. SP and NS are easy, just a different level of dirty 

Oliver

[chuck geiger]

[muktown128]

As a retired paint chemist with over 35 years experience formulating industrial coatings that included formulating yellow colors like Caterpillar Yellow and New Holland Yellow, I may be able to provide some additional insight.

1. Unless using ultradurable pigments (typically inorganic mixed metal oxides), color will change (fade) over time when exposed to UV light. 
2. Color should not vary when applied over different colored primers, if opacity is achieved.  Ideally, topcoats or colored basecoats should be applied over a neutral color primer (light gray or buff), but sometimes yellow topcoats are applied over dark gray or black primers.  In those cases, color can be affected more if the topcoat film thickness is not sufficient to achieve opacity.  Sometimes color-keyed primers are used if the topcoat has poor opacity or to formulate a topcoat with a lower pigment level in order to reduce cost.
3. Colors can be matched using different combinations of pigments.  This could result in metamerism.  One way to detect metamerism is how colors match under different light sources.  A metameric match occurs when 2 colors match visually under one light source (i.e. daylight), but not under a different light source (i.e. fluorescent).  Matching colors with different combinations of pigments could result in spectral curves that don't match exactly, but rather cross at certain wavelengths.  Comparing spectral curves to see if the curves cross one another is another way to check for metamerism.  It is likely that the pigments used in model paints are not that same as the pigments used in industrial coatings.  Industrial / automotive grade organic pigments are intended for applications where exterior durability (resistance po color change) is a key requirement.  Organic pigments used for model paints typically would be less durable and less expensive than exterior durable organic pigments. 
4. I've seen some industrial coatings with poor durability (excessive color fade) occur where lower quality/less durable pigments were used.  I suspect this would explain the differences in color on the Santa Fe units that were fairly new.  This would not be too surprising if coatings were from different paint manufacturers.  I would not be surprised if the color match of these coatings was metameric to one another.
5. Changes in environmental regulations did eliminate the use of certain pigments (lead and hexavalent chrome (chrome VI) containing pigments).  Some companies like Caterpillar, which wanted to maintain durability, chose to change their yellow color to comply with the new regulations, while other companies like John Deere, chose to keep the bright yellow color at the expense of cost and possibly durability.
6. Government regulations regarding VOC impacted the resins and solvents used in formulating coatings.  I'm not aware of any changes in pigmentation or color due to VOC regulations.  Exterior durability (mainly gloss retention) is very dependent on the resins used in coatings.  Changing resins could have a significant effect on exterior durability. 

Fun fact - the company I worked for supplied coatings to EMD/Progress Rail and Micro-Trains. 
Yes, I did have access to the paint formulas supplied to these companies. 
No, I did not take copies of these formulas with me when I retired.

Scott

[nkalanaga]

Interesting that EMD/Progress Rail and Micro-Trains bought paint from the same supplier.

[muktown128]

The products for Micro-Trains were actually developed by a company acquired by the company I worked for.  The company I worked for acquired many smaller paint companies over the years.

[peteski]

The products for Micro-Trains were actually developed by a company acquired by the company I worked for.  The company I worked for acquired many smaller paint companies over the years.

Sounds like RPM with Rustoleum being one of their main consumer paint brands. 

[muktown128]

@peteski
LOL!  I didn’t work for RPM.  However, Rustoleum has a plant not too far from where I worked. I know several people that have worked at Rustoleum and where I worked.  The company I worked for is headquartered in Pittsburgh and is in the top 2 of global coatings manufacturers.

[peteski]

@peteski
LOL!  I didn’t work for RPM.  However, Rustoleum has a plant not too far from where I worked. I know several people that have worked at Rustoleum and where I worked.  The company I worked for is headquartered in Pittsburgh and is in the top 2 of global coatings manufacturers.

That was just a semi-educated guess based on the fact that RPM took over several smaller independent (hobby) paint manufacturers like Floquil and Testors.  In light of what you mentioned, it was probably PPG. 

[muktown128]

That was just a semi-educated guess based on the fact that RPM took over several smaller independent (hobby) paint manufacturers like Floquil and Testors.  In light of what you mentioned, it was probably PPG. 

Winner, winner, chicken dinner!!! 
Thanks for playing along 

[peteski]

Winner, winner, chicken dinner!!! 
Thanks for playing along 

That was fun. 

Yes, PPG is a coatings giant. 

[randgust]

"Santa Fe's Diesel Fleet" by McMillan has some great shots of the yellowbonnet being applied in the paint shop, and all the taping process involved to get the herald on the nose and the "Santa Fe" on the sides.  Unbelievable masking job, not stencils.   The yellow was the base coat.   Yellowbonnet evolved simply by not painting the red over the yellow, not striping the yellow around the red as it would seem.  Every once in a while you'd see a weathered passenger F with the red worn through somewhere and yellow visible underneath.

We had four patched CF7's on our local shortline starting in the mid 80's, and the original paint on all of them was beyond bad.   Considering they had only had about 10 years of ATSF service at most, the condition of the paint and primer was rather appalling; the rusting and fading was rapid.
https://www.rrpicturearchives.net/showPicture.aspx?id=546832
https://www.rrpicturearchives.net/showPicture.aspx?id=1190241

Everything I've read about the CF7's indicates it was a real penny-pinching exercise at Cleburne, I've always concluded that included a 'get er done' in the paint booth with insufficient surface prep, primer, and no clearcoat.  Anybody else that started over with paint had much better results.   Stuff that got painted in Topeka or Barstow seemed to hold up much better.   But you can see that the entire paint job failed on the CF7's.