The engine in the first Bighorn (Gold one in all the adds) was white in all the photos that I have seen. Remember there is usually an exception to the rule. The only time that my dad worked for a company that bought an entire new fleet of trucks was in 1983. Out of over 40 brand new 83 Freightliners, two of the 3406 Cats were white, the rest were all yellow. The old K100 that my dad drove had a green Detroit, while some were white. The company that I worked for back in the early 90s painted every Mack engine that was rebuilt grey, even the gold ones. Not to mention engine swaps, new replacements, etc. The point is; when modeling, there is room for variation unless you are modeling an exact replica.