The more common accidental coloring is the result of excavating silica sand deposits contaminated with iron and copper oxide impurities. Mined silica sand for glass making needs to be extensively washed and cleaned in order to produce good quality glass. This simply wasn't required for insulator production. Varying degrees of iron and copper oxides produce the vast array of aquas and greens we see in insulators. These insulators are usually from glass companies using bottle glass to press insulators.
Cobalt oxide is added in very small amounts to glass ingredients to produce the cobalt blue color.
It produces cornflower blue in very very tiny amounts. Amber is produced with sulfur and iron oxide. Amber and cobalt bottles were intentionally made for medicines and liquids which required some UV protection. Some blue and amber insulators were marketed in the early 1900's as line markers and would have been intentionally colored. These H.G.Co. and Hemingray insulators wound up being various shades of yellow, amber, red amber, brown amber and also electric blue, peacock blue, and various cobalt blues.