The general rule is to replace coolant every other year or 30,000 miles, whichever comes first. Coolant with a reddish cast or milky swirls signals a rust problem in the cooling system. Flush the system thoroughly and refill with fresh coolant. If you detect black swirls or oil floating on the surface, engine oil may be leaking past a damaged head gasket. If the 4x4 is equipped with an automatic transmission and the coolant has turned a milky pink, the transmission cooler is probably leaking.
Engine oil is a clear amber when new, though sometimes it appears a smoky olive.
Under normal conditions, the color will darken to a golden brown after a few hundred miles of use. Brown or black stains underneath the front half of the truck are oil. If your 4x4 has a manual transmission, you may see a leak under it as well. Oil leaks are most noticeable on older trucks, and are mainly due to component wear. Other common causes are worn gaskets, faulty seals, or even a loose oil filter.