Dynamically reconfigurable structural colors are promising materials for new smart optical systems. However, improved reflected color quality (e.g., saturation, optical contrast, angular invariance) and larger tuning range/sensitivity are needed. Here, we demonstrate a vibrant, actively tunable system which meets these needs via coupling broadband plasmonic resonators to a responsive polymer film. Our structure consists of near-percolation gold nanoislands deposited on a poly[methyl methacrylate] (PMMA) spacer above a gold mirror, forming a Fabry-Pérot nanocavity. Broadband absorption in this system creates vivid reflected colors, while the polymer spacer enables continuous tuning over a wide color space. By exploiting swelling effects in PMMA, we show fast, reversible color switching in response to organic vapors. Our sensitive optical structure amplifies small vapor-induced changes in the spacer thickness, enabling naked-eye detection of changes as small as 10 nm. Additionally, optical absorption >99% yields modulation contrasts up to 80:1, opening the door to ultra-sensitive on-chip signal measurements, complementing the visual colorimetric readout. This structure has immediate implications for colorimetric bio/chemical sensing and may also find application to reflective displays and flexible/adaptive optical coatings.