| Oxides | Xystal color | band gap (eV) | Oxides | Xystal color | band gap (eV) | Oxides | Xystal color | band gap (eV) |
| 2+ | 3+ | 4+ | ||||||
3d0 | Sc2O3 | white | 6 | TiO2 | white | 3.2 | |||
3d1 |
| Ti2O3 | violet | 4.08 | VO2 | blue-black | 0.7 | ||
3d2 | TiO | dark | 0 | V2O3 | black | 0.2 | CrO2 | black | 0 |
3d3 | VO | black, grey, or green | 0 | Cr2O3 | green | 3 | MnO2 | gray to black | 3.4 |
3d4 |
|
|
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3d5 | MnO | green | 3.6-4.3 | Fe2O3 | red-brown | Xystal 2.0/film 2.7 |
| ||
3d6 | FeO | black |
|
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3d7 | CoO | grey or olive green | 2.4 |
|
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3d8 | NiO | green | 3.0-4.3 [Sawatzky1984, Huefner1985] |
As we can see that the color does not correlate with band gap, except for the case that the material is always black when the gap is lower than 1.8 eV. This is because the optical gap we normally refer is actually charge transfer gap which bare huge absorption strength which color properties can be determined by the excitation that has much less absorption strength, e.g. on-site forbidden excitation. See this post.
[Sawatzky1984] G. A. Sawatzky and J. W. Allen, Phys. Rev. Lett. 53, 2339 {1984).
[Huefner1985] S. Huefner, Solid State Commun. 53, 707 (1985).