Warning - this can really waste a lot of your time!
The rules of the game will be immediately obvious to anyone who has done Soduku. Basically the grid is divided into 16 quadrants and each row, column and quadrant must contain one and only one of each of the 16 possible colours.
You can select or unselect colours from the 16 possibilities for the current cell using the left mouse button. If you know that a given cell can only contain one colour, there is a right-mouse shortcut menu to select a colour. Also there is an option on that menu to clear a cell back to all 16 possibilities if (when) you get it wrong.
Grids can be saved and loaded to an XML file (with the extension .xhx) and there is one example included.
This program demonstrates using flag values (integers defined as powers of two) to set or unset bits in a 32 bit integer.