I made this simple puzzle a couple of years ago. There are three colors which appear in every row and every column. Moreover, every color appears 5 or 6 times in total. Find the color of the square with the question mark.
Top right is fairly obvious, the 3rd row needs a red and green, and the red can't go in column 2.
Top row and last column now need red/green, and first column, last row need a blue, so the blues go.
Now we have 6 blues, and so we must have 5 reds and 5 greens. As top row/last column both need red/green, this gives us 5 reds, so ? is green.
Answer for the question mark:
If we look at the right column, we can't get any more blues otherwise the red and green couldn't fit in. These are marked with a diagonal line:
Then the bottom row must have blue as shown:
We can apply the same argument now to the third column and then the second row:
And to the second column and the third row, but in red:
So now we can find the green in the third row:
And the blue in the second column:
We can't have another blue in the top row, so the remaining rows columns apart from the second must each have a red and a green:
So there's exactly one green in the third column and exactly one more in the fourth. Since there is at least 5 greens, the top-left square must be green:
Because the first row and the third column each need to have one red and one green, we have the following:
And then we have the ambiguity of which colours go where, and this cannot be resolved.