very off topic question

@BlueStarAerospace Yeah I'm prolly recalling the same graphic renderings you've seen, one of the most notable being the image a top the Wikipedia page "Dleta (Rocket Family)". Prolly just drawn that way out of ease (hesitate to call it laziness as they musta spent some time making that diagram).
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Though I've been unable to verify the color of an uninsulated Delta IV tank (without the orangish foam coating), I can say that the Delta II & III blueish color appears to be due to the primer paint color originally used on these "Extended Long Tank Thor" boosters! Originally (prolly in the late 60's early 70's) they painted over top the primer with a white coat of paint but then decided to forgo that step and just leave the blueish primer coat as the main color to save weight. This also helped them better identify where ice formed (over the liquid oxygen sections of the tank) but they also just liked the color as well so it all worked out!
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WB BTW after your computer hiccups and thinks for making me look into this topic! More useless information I'll never ever need but it's always fun to learn some stuff!

most of the time I see a Delta IV drawn in the blue-green its usually with the Delta II (and its variants) next to it so I think @Toinkove is right and the person who made the drawings just assumed the paint continued with the Delta IV

@deepfriedfrenchtoast I kind of doubt this because by the time the Delta IV was proposed (in 1994) they would have known that painting over the foam was a counterproductive step that only added more weight to the vehicle (that lesson was learned with the space shuttle in the early 80’s). I suspect whoever rendered the drawings that depict the Delta IV with the same blue green painted tanks that previous Delta launch vehicles was just uninformed.

I think the blue paint was ment to protect the orange insulation from UV light, though it stoped being used because painting the whole first stage added too much weight.