Each problem draws its own single-series bar chart - months, days, colours, fruit or sports along the bottom, four to six bars tall - and asks something about it: how many for one category, how many more one has than another, the total of every bar added together, or which category has the most. Three of the four are ordinary addition and subtraction with the numbers read off a picture instead of written down; only reading a single bar is pure reading.
Every bar ends exactly on a labelled gridline, never between two of them. A bar a student had to estimate would have no single right answer, so every value on this sheet is drawn to land precisely on a line the chart already shows the value of.
Find the bar for the category you want, follow its top across to the scale on the left, and read off the number the gridline there is labelled with.
So there is only one correct reading. A bar that stopped between two gridlines would leave a student guessing whether the answer is the lower value, the higher one, or something in between - and a key can only print one of them.
No - the bars print in grey with a black outline, on purpose. A worksheet is printed in black and white far more often than not, and a chart that only makes sense in colour fails the moment it comes out of a black-and-white printer.
Second and third grade (2.MD.D.10, 3.MD.B.3) introduce reading and comparing a bar graph; by third grade, students are also expected to solve one- and two-step problems using the data it shows.