Highcharts supports three color formats primarily: hex (#FFFFFF), rgb (rgba(255,255,255)) and rgba (rgba(255,255,255,1)). If any other format, like 3-digit colors (#FFF), named colors (white) or gradient structures are used in for example a heatmap, Highcharts will fail to interpolate and will instead use the end-color with no interpolation applied.
We've chosen to preserve this limitation in order to keep the weight of the implementation at a minimum.