Unfortunately, you did not specify which oil or which plant you are talking about in your question. In principle, it is worth having even dark oil analyzed, because there are many different causes for a change in color:

• Oils cannot become lighter than the raw materials during production, even if colorants are added. They only become darker.
• The color of a lubricant or hydraulic medium is not an indication of the quality of the oil, base oils or additives used.
• Depending on the viscosity and origin of the base oil, the following usually applies: thin metalworking oils are water clear, hydraulic oils are golden yellow, and high-viscosity gear oils are brownish.
• Often, base oils in the lubricant refinery are treated with bleaching earth after dewaxing (wax removal) so that the color variation between different production batches due to the origin of the oil does not fully show through.
• Fully synthetic oils or oils from the hydrocracking production process are mostly water clear. Additives are dark brown to black. Depending on the oil type, 0.25 to 25% ofadditives are added. The saleable finished products then become light yellow to dark brown due to the oil-soluble additives.
• Often oils are additionally marked with a typical dye with a reddish, greenish or bluish tone, which is not related to the quality.
• Oils darken in use primarily due to oxidation. The more advanced the oxidation , the darker the oil becomes. 
• With some oil types, whose criterion for an oil change is almost always oxidation, a barely perceptible change in color can already indicate that an oil change is necessary. In the case of oils from screw or refrigeration compressors, transformer or heat transfer oils, and hydraulic media from low-pressure hydraulic systems, the color index often reveals the oil condition. However, an oxidation-related color change can hardly be perceived with the human eye, but only with instruments such as the reliable WaveCheck 300.
• Oils stored in transparent containers often change color due to the exposure to light. Especially if manufacturer-typical color indicators or fluorescent substances are added during production, the UV radiation can change the appearance to such an extent that this suggests a different oil.
• When the oil flows from a container, it has room temperature of less than 25° C; when the sample is taken, it should be at operating temperature, usually about 50° C. During the flow process, the oil stream is thicker or thinner and consequently appears differently colored. When color comparisons are made, oils should always be compared at the same temperature and in the same container types with the same film thickness.
• The color of hydraulic and gear oils can also change due to black soot pigments caused by diesel effect due to poor air-release properties.
• Corrosion particles also do not have a metallic copper or silvery shine. Additive reactions produce oxides that uniformly color the oil black, even if they themselves were formed from copper, iron or aluminum abrasion.
• When mixed with water, light oils become milky cloudy and dark brown or black oils become gray.
• Sudden color changes, e.g. into red or blue, indicate mixing. If a yellow oil has a reddish glow, it may be mixed with an ATF oil. These are usually intensely red in color. Even residual amounts of ATF color the entire oil red.
• The color of an engine oil can change after just a few kilometers. Diesel engine oil turns black after a short period of use. This is a good sign that sufficient detergent additives are present to keep the soot from the diesel combustion process in suspension.
• Engine oil from gasoline or gas engines often only turns dark brown, because less black soot is produced when ignited by spark plugs. The brown coloration is caused by aging of the oil due to temperature, long time of use or reaction products from oil additives and other impurities.