Fix “Invalid value [color]” on Google Merchant Center

Invalid colour is most often Google's classifier rejecting brand-marketing colour names. "Midnight Whisper", "Sahara Sunset", "Coastal Dream" — these are marketing names, not colours. Google can't map them to its colour taxonomy for filtering, so the field gets flagged as invalid.

The strategic compromise that works: keep the marketing colour name in the title for brand positioning, use standard colour names in the colour field. "Midnight Whisper Wool Coat" in title, "Navy" in colour. Users searching the marketing name still find the listing via title; users filtering on colour find it via the standard colour value.

For multi-colour items, the slash separator is the rule: "Red/White/Blue", up to three colours. Comma-separated values, semicolons, "and"-joined lists all get rejected. Beyond three colours, use "Multicolour" as the single value.

For colour codes (hex values, RGB, brand-specific codes), the field is for human-readable colour names, not codes. `#1A2B3C` doesn't work — use the standard name closest to the hex. Some categories where colour matters precisely (paint, dye, makeup) might benefit from including the brand colour code in description while keeping the colour field standard.

The Optimiser can map brand marketing colour names to standard equivalents at scale — feeding it your colour-name mapping table and applying it as a feed-generation rule.