colors2.java

public List<String> getCommonColors(String blueprintId, List<String> printProviders) throws IOException {
    List<String> commonColors = new ArrayList<String>();
    Map<String, Boolean> colorsMap = new HashMap<>();

    for (String printProvider : printProviders) {
        VariantsResponse variantsResponse = getVariants(blueprintId, printProvider);
        List<Variant> variants = variantsResponse.getVariants();
        for (Variant variant : variants) {
            Options options = variant.getOptions();
            String color = options.getColor();
            if(!colorsMap.containsKey(color)) {
                colorsMap.put(color, true);
            } else {
                colorsMap.put(color, false);
            }
        }
    }

    for (Map.Entry<String, Boolean> entry : colorsMap.entrySet()) {
        if (entry.getValue()) {
            commonColors.add(entry.getKey());
        }
    }

    return commonColors;
}