Place lights in some unshaded cells so that every unshaded cell is illuminated. Lights illuminate the cell they’re in as well as all cells seen in a straight line horizontally or vertically up to either the first shaded cell or the edge of the grid. Lights may not illuminate each other. A number in a shaded cell counts the lights orthogonally adjacent to that cell. Each light is either Red, Green or Blue. A clue in an unshaded cell gives the colour resulting from all lights illuminating that cell.