Yes, several factors beyond stoichiometry and ball milling conditions can influence the color of your synthesized material, even when using the same synthesis route. Here are some possible reasons for the color variation in your samples:

1. Moisture Content and Drying Effect

• Drying your precursors overnight may change the water content or hydration state of the materials, which can impact the final phase composition, particle size, and electronic structure.

• If the precursors absorb moisture before synthesis, they may introduce slight compositional variations, affecting oxidation states and color.

2. Oxidation State Variations

• Ball milling can induce oxidation or reduction reactions, especially in transition metal oxides.

Exposure to air during milling or storage can lead to partial oxidation or reduction, leading to color changes.

• If drying influences oxidation states by removing bound water or oxygen, the final product might differ in color.

3. Particle Size and Morphology

• Ball milling affects particle size, and even minor variations in milling efficiency (due to small changes in ball loading, environmental conditions, or mill wear) can result in different particle sizes.

• Finer particles may scatter light differently, altering the apparent color.

4. Contamination from Milling Media

• If the milling media (balls and jar) introduce minute contamination over repeated use, trace elements can alter the material’s color.

• Even slight wear from the milling container can introduce elements that modify the color.

5. Thermal History and Phase Formation

• If drying your precursor materials overnight alters their phase composition or introduces minor differences in precursor reactivity, the final synthesized material may contain slightly different phases, affecting color.

• Minor variations in crystallinity or amorphous content due to differences in processing history can also affect optical properties.

6. Environmental and Handling Conditions

• If the synthesis is performed under slightly different ambient humidity, temperature, or exposure to light, it could impact the intermediate and final products.

• Small variations in exposure to air or moisture post-milling can lead to surface oxidation or hydroxylation, influencing color.

7. Surface Chemistry and Adsorption Effects

• If your samples are exposed to different atmospheric conditions (humidity, CO₂, or other gases), they may adsorb small amounts of species that subtly affect electronic structure and color.

• The surface of the powder can also undergo slight hydration or carbonization effects, which may shift its color.

Next Steps: How to Verify the Cause

• XRD Analysis – Check for phase purity and crystallinity changes.

• XPS or UV-Vis Spectroscopy – Determine oxidation state and band gap variations.

• SEM/EDS Analysis – Identify any unexpected elemental contamination.

• Compare Drying vs. Non-Drying – Synthesize two batches with and without precursor drying and compare the final colors and properties.

• Check Milling Media – Ensure no contamination or wear is affecting the results.

I hope this helps?