Visible‐Light Mesoinoic Oxidopyrylium Ylide Photoswitches for Multicolor Photochromism and Dual‐λ Synergistic Photoclick Reactions with Dibenzo[ <i>b</i> , <i>f</i> ][1,4,5]Thiadiazepine‐Dioxide

) and dual-λ orthogonal photo-control for protein labeling, outperforming the benchmarks, e.g., bicyclo[6.1.0]non-4-yn-9-ylmethanol (BCN-OH) and dibenzothiadiazepine (DBTD). The P-type photoisomerization mechanism of the DIO⇌PY system is clarified for the first time via theoretical calculations together with experimental validation, revealing competitive singlet/triplet diradical pathways during photo-ring-opening/closure cycles. Transition-state analyses rationalize the superiority of DBTDD over DBTD in the [5+2] cycloaddition, offering mechanistic insights and principles for advancing the dual-λ photoclick platform.