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What color is flex fuel?

Flex fuel, also known as ethanol-gasoline blends, refers to fuels that contain a mixture of gasoline and ethanol in varying percentages. The most common flex fuels are E85 and E15. E85 contains 51% to 83% ethanol, while E15 contains just 15% ethanol. The color of flex fuel depends on the exact ethanol content, but generally ranges from clear to yellowish in appearance.

Ethanol Content and Color

Ethanol is a clear, colorless liquid made from renewable sources like corn and sugarcane. When mixed with gasoline at low percentages, such as 10-15%, it does not significantly alter the light yellowish color of gasoline. However, at higher blend levels like E85, the ethanol content imparts more color and the fuel takes on a yellow, almost amber-like hue.

Here is a rough overview of how ethanol content affects the color of flex fuels:

Ethanol Content Typical Color
0-10% (E10) Clear to light yellow
15% (E15) Light yellow
51-83% (E85) Yellow to amber

As you can see, the more ethanol blended into the gasoline, the darker the yellow color becomes. E10 and E15 look similar to pure gasoline, while E85 takes on a noticeably yellowish-amber shade.

Factors Affecting Flex Fuel Color

While ethanol content has the biggest impact on color, there are a few other factors that can alter the exact shade and intensity of yellow in flex fuels:

  • Gasoline composition – The type and quality of the gasoline used in the blend can impact color. Gasoline itself ranges from clear to light yellow or even orange tinted.
  • Ethanol feedstock – The source of the ethanol, whether corn, sugarcane, or another biomass, can subtly influence color.
  • Dyes and markers – Trace amounts of dyes or markers may be added to help distinguish flex fuels from traditional gasoline. These can make the fuel slightly darker.
  • Oxygenates – Other oxygenate additives like MTBE may be included in small amounts, potentially altering color.
  • Contaminants – Dirt, water, or other contaminants introduced during handling and storage could cloud or darken the color.

So while the ethanol content has the most direct impact on flex fuel color, the exact yellowish-amber shade can vary somewhat based on these other factors. Fuel suppliers try to maintain consistency, but some color variation is to be expected.

Yellow Color as a Safety Indicator

The yellowish color that emerges at higher ethanol percentages serves an important safety purpose. It helps distinguish flex fuels like E85 from regular gasoline, signaling to drivers that the fuel requires special handling and is only compatible with flex-fuel vehicles.

Without this visual cue, there is risk of misfueling a conventional gasoline vehicle with higher ethanol blends like E85. Even small amounts of E85 in a gasoline vehicle can cause performance issues and engine damage. The yellow color is a built-in warning sign that helps prevent costly mistakes.

To reinforce this, E85 pumps are also required to have special labels identifying the fuel as not suitable for all vehicles. So the yellowish hue provides an immediate visual indicator, while the labels provide confirmation.

Color Variation in Different Lighting

The lighting conditions can also impact how dark or light the yellow color of flex fuels appears:

  • Daylight – Natural daylight best represents the true color. E85 will appear yellow to amber gold.
  • Indoor lighting – Fluorescent or LED lights may lend a slightly paler, diluted look compared to daylight.
  • Nighttime – At night or in dark surroundings, the yellow color may be less discernible unless directly illuminated.

So the intensity and shade of the yellow color can shift subtly depending on the ambient lighting. Daylight shows the truest color, while low light conditions may make it harder to distinguish unless directly lit by pumps or overhead lights.

Effects on Engine Components

The ethanol content in flex fuels can also impact the color of engine components it contacts over time. Here are some potential effects:

  • Fuel lines – Ethanol may cause yellowish staining of rubber fuel lines and gaskets over time.
  • Plastic and fiberglass tanks – Discoloration or hazing is possible in plastic and fiberglass fuel tanks and containers.
  • Aluminum parts – Can slowly corrode or oxidize aluminum fuel system parts, resulting in yellowish or white powdery deposits.
  • Steel parts – May lead to increased rust and corrosion of steel fuel tank and component surfaces over time.

These effects stem from ethanol’s solvent properties and propensity to absorb water compared to gasoline. Proper component materials compatible with higher ethanol concentrations are used in flex fuel vehicles to minimize these issues.

Measuring Flex Fuel Color

While the yellow color of ethanol blends is clearly visible, fuel suppliers also use technical measurements to quantify the color for quality control purposes:

  • ASTM Color Scale – The ASTM D1500 color scale is used, ranging from 0.5 to 8 to describe fuel color. E85 typically ranks 2-4.
  • Saybolt Color – The Saybolt Color test measures color intensity on the Saybolt Universal Scale. E85 can range from -16 to +25 Saybolt.
  • Spectral Analysis – Laboratory spectrophotometers can precisely measure light absorption across color wavelengths.

These analytical tests help verify that flex fuel ethanol content aligns with the desired specifications and color range. However, the simple yellowish tint observed by eye serves as the most fundamental indicator of ethanol content.

Typical Flex Fuel Colors

Here is a summary of the typical colors exhibited by common ethanol-gasoline blends:

Fuel Type Ethanol Content Typical Color Range
E10 (Gasoline) 0-10% Clear to light yellow
E15 (Gasoline) 15% Light yellow
E85 (Flex Fuel) 51-83% Yellow to amber

So in summary:

  • E10 has little to no color tint
  • E15 has pale yellow tint
  • E85 has distinctive yellow to amber color

This color progression from clear to light yellow to full yellow-amber directly correlates with increasing ethanol content in flex fuels.


The color of flex fuel provides a useful indicator of the ethanol content in the gasoline-ethanol blend. While pure gasoline is clear to light yellow, higher ethanol mixtures like E85 take on a progressively darker yellow-amber color. This results from the inherently colorless ethanol mixing with and diluting the yellowish gasoline components. The yellowish tint also serves as a safety warning, signaling the fuel is not suitable for all vehicles. So next time you see a yellow colored fuel at the pumps, you can confidently conclude it is a flex fuel with significant ethanol content.