Maximum two: You can only place up to two blocks of the same color directly next to each other (horizontally or vertically). Three in a row is invalid.
Equal division: Every row and every column must contain an equal number of blue and green blocks.
Equal signs (=): Two empty adjacent blocks with an = sign between them must always receive exactly the same color.
Pure logic: You never have to guess. Look closely at the rules to deduce the missing colors.