Can group 1 form coloured compounds?Watch
This is because group 1 forms ionic bonds and the ions of group 1 are colourless.
When transition metal complexes absorb electromagnetic radiation of a certain energy, it causes an electron in a d-orbital to transition to a higher-in-energy d-orbital, and thus be in an excited state (these are called d-d transitions)
This electron can de-excite to its ground state, and emit electromagnetic radiation in the process; most of the time, this radiation comes under the visible region, hence these complexes appear coloured
Of course, in order for this to happen, d-orbitals must be vacant/present for electrons to transition to; the problem is that empty d-orbitals are too high in energy relative to a Group 1 metal's s-orbitals, and so these transitions simply do not occur
Transition metal complexes on the other hand don't have this problem as they're, well... transition metal species, and they're kind of fundamentally defined in terms of them having incomplete d-orbitals, hence these transitions can occur
Alas, even some transition metal complexes give rise to pretty weak-intensity colours, and this is further down to rules associated with d-d transitions... So even then, not everything gives rise to colour!
(I’m doing GCSE content at the moment)