* Returns the sequence feature colour rendered at the given column position,
* or null if none found. The feature of highest render order (i.e. on top) is
* found, subject to both feature type and feature group being visible, and
* Returns the sequence feature colour rendered at the given column position,
* or null if none found. The feature of highest render order (i.e. on top) is
* found, subject to both feature type and feature group being visible, and