Because consumers expect oil to be Amber. Learned it from MolaKule. I'll see if I can find the post.
Quote (Mola):
Most oils are dyed "bronze" which we call Amber, because oils are expected (by most of the general public) to look bold and amber.
Redline uses red and orange dyes to differentiate its products, GC and Schaeffer's use blue/green, and of course, Royal Purple uses a specialized red/blue dye combination.
Most additives, right after reaction and processing, are clear. Bronze and/or red dyes are added to differentiate the additives. Sometimes the metals in certain additives, such as MoDTC and calcium and magnesium sulfonates, tend to darken the additive, so the above dyes are added to mask the darkening.
ATF's and most PS fluids are dyed red to detect leaks.
AC compressor fluids are dyed with a fluorescent dye so a blacklight source can be used detect leaks as well.
And
It depends on the additive supplier. Some supply Dispersives for example, that are clear "water white."
On the other hand, some suppliers supply MoDTC almost clear with a greenish tinge, while others color it with a reddish-brown dye.
The darkest additives (dark-reddish-brown) have to be the detergents, such as the calcium and magnesium sulfonates.
Even if you take all the additive amounts required per quart, and even if the majority of additives are colored, the final quart of oil that started out as as water white will only become slightly amber.
The rest is usually dye.
End Quote