سلام

برخی مواقع ترکیبات فلزی دارای دو یا چند رنگ هستند که بسته به شرایط سنتز ممکنه یکی از اون رنگ ها حاصل بشه. مهم ساختار و خواص یکسان هستش. در مورد اکسید کادمیوم هم همین مسئله هستش یا در مورد بقیه اکسید های فلزی

Provided both products are identical and pure the size of the microcrystals may influence of the colour appearance - larger crystals absorb more light and the absorption is tailing more to the visible region. You can try to measure diffuse reflectance spectra on your materials to see the difference..

Yellow substances generally absorbs in the near UV - Violet region. So the yellow color do not arise from the absorption at their maxima, but from the band tails that extend toward the red.  The concentration (or the density in the solid) produces a shift in colour from yellow, orange, red, brown, etc. depending on how long the tail extends, due to the sensitivity of the eye which is maximum at green.  And  some times, small quantity of impurities (i.e. other oxidation states) that absorb in green can contribute to the brown colour of a substance that just absorb in violet-blue. 

If you are sure to have the same product (Do you have crystals or powder), sometimes in the crystallization process there are impurities or different forms to coordination. Generally, the solvent present inside the layer at the crystal could change the color.