Indeed, TEA-N→O absorbs at 198 nm (in hexane), which means that it transmits in rest of UV plus the overall visible spectrum and consequently, it has colorless appearance.
But dilution in polar solvents (such as TEA) and especially in presence of oxygen, causes absorption blue shift that may reach up to the near UV spectrum, depending on the strength of intermolecular interactions with the solvent.
On the other hand, the clearly visible spectrum is 380-740 nm. However and under certain circumstances, humans can percept wavelengths that are out but near the edges of the visible spectrum; but weakly and unclearly.
Thus, TEA-N→O diluted in TEA absorbs in the near UV due to the absorption blue shift; as a consequence it transmits in the complementary spectrum, which is unclear brown that is the additive color of white, red and black. But in the high dilution and due to the high dilution, TEA-N→O (as well as all amine N-oxides) has an unclear yellow-orange-brown appearance, depending on the storing time.
In daily life, the same phenomenon occurs when washing white clothes, either by hand or in the washing machine. The washing waste water has a weak and unclear grey-brown coloration (complementary transmision color), due the wavelength absorption of dirtiness in the close-near UV spectrum.
Ultraviolet absorption spectra of trimethylamine N-oxide, Journal of Molecular Spectroscopy, 20(3), 226-232, (1966)https://www.sciencedirect.com/science/article/abs/pii/0022285266900245
The Singlet-triplet Absorption Spectra of Heterocyclic Amine N-Oxides (I), Bulletin of the Chemical Society of Japan, 45(11), 3282-3286, (1972)https://www.journal.csj.jp/doi/pdf/10.1246/bcsj.45.3282
Human infrared vision is triggered by two-photon chromophore isomerization, PNAS, e-article in early edition, (2014) https://www.pnas.org/content/pnas/early/2014/11/25/1410162111.full.pdf