These Sentinel-2 images show the extent of the recent flooding near the border between Arkansas and Missouri on Apr 24, 2017 (36°30'N 90°38'W). The left image is a natural color composite (similar to what our eyes could see if we were in orbit). The right image is a color representation of the Normalized Difference Water Index (NDWI).
The NDWI is derived from the Near-Infrared (NIR) and green channels (McFeeters 1996):
NDWI (McFeeters) = (Green-NIR) / (Green+NIR)
The NDWI is efficient to detect surface water because these surfaces have a very low reflectance in the NIR region of the spectra - in contrast to the vegetation which is characterized by a high reflectance in the NIR. In the Sentinel-Playground, the NDWI is represented using a clever colormap, which is blue if the index is high, and green if the index is low.
The contrast between the vegetation surfaces and the water surfaces is evident if we use a near-infrared composite (where the red channel of the picture is replaced by the near infrared channel from the satellite) instead of the natural colors:
This NDWI should not be confused with another NDWI (also named Normalized Difference Moisture Index, NDMI), which is derived from the near-infrared (NIR) and shortwave infrared (SWIR) channels (Gao 1996):
NDWI (Gao) = (NIR-SWIR) / (NIR+SWIR)
The latter is supposed to reflect liquid water content of vegetation canopies. McFeeters paper was published in January 1996 and Gao paper in December 1996, hence it is likely that Gao was not aware of McFeeters' definition!
To highlight the extent of the flooding we can use another Sentinel-2 image, which was acquired last year in the same period of the year:
Some dark green patches in the image above are forests that might be flooded as well, but the NDWI does not allow the detection of floodings under forest canopy.
McFeeters, S. K. (1996). The use of the Normalized Difference Water Index (NDWI) in the delineation of open water features. International journal of remote sensing, 17(7), 1425-1432.
Gao, B. C. (1996). NDWI—A normalized difference water index for remote sensing of vegetation liquid water from space. Remote sensing of environment, 58(3), 257-266.