A blue airport symbol on a sectional means the airport has a control tower, and a magenta symbol means it does not. A star means the tower is part time, so the hours are in the Chart Supplement. Field elevation in the data block is feet MSL, and a runway figure of 40 means 4,000 feet because those digits are hundreds of feet.
Color, then the star
The color answers one question: does this airport have a control tower? Blue means yes. Magenta means no. The color is not a seaplane-base code, and it is not a heliport code. It is not a runway-length code. Blue does not mean the runway is longer than 5,000 feet, and magenta does not mean the runway is shorter.
A star beside the tower information means the tower operates part time. Check the Chart Supplement for the hours before you assume anyone is in the cab. The color still only tells you that a tower exists. It does not tell you the tower is open on the afternoon you fly. Magenta does not mean the airport is closed to all aircraft. It means you will not find a control tower there.
Lighting is a different mark. An L in the data block means runway lighting is available. An asterisk with the L, written *L, means lighting limitations exist, and those limitations are explained in the Chart Supplement. *L is not the part-time-tower star, and it does not by itself mean the lights are only pilot controlled. Keep the star on the tower and the asterisk on the lights.
The line that holds the elevation
Reading a typical block from the name downward, you find the airport name, then the frequencies, then a bottom line that starts with the field elevation. That elevation is feet MSL. It is the first number on the line that also carries the lighting code and the longest runway. It is not a frequency, and it is not scaled in hundreds of feet.
FAA-CT-8080-2H Figure 26 shows the pattern on Jamestown Regional, a magenta airport and therefore non-towered. The block includes ASOS 118.425, then a bottom line that reads 1500 *L 65, then 123.0 with a C in a circle. Read 1500 as the field elevation, 1,500 feet MSL. Read *L as lighting limitations, so you open the Chart Supplement. Read 65 as the longest runway in hundreds of feet, which is 6,500 feet. Read 123.0 with the circled C as the CTAF. The ASOS frequency is the weather broadcast. Do not announce traffic on the ASOS, and do not treat 118.425 as the field elevation.
The circled C is the CTAF, whether it sits on the bottom line or next to a frequency above it. AWOS and ASOS are automated weather. CT is a control-tower frequency at a blue airport. Swapping the weather frequency and the CTAF is a common miss because both are printed as megahertz. The CTAF page covers how you use the advisory frequency. Runway numbers, as opposed to runway length, come from magnetic heading and are covered with runway numbers.
A runway length of 40
The longest-runway figure is in hundreds of feet. A data block that prints 40 means 4,000 feet. It does not mean 40 feet, and it does not mean 40,000 feet. Multiply the printed digits by 100. The same habit fixes the Jamestown 65: 65 times 100 is 6,500 feet. Field elevation on that block is already in whole feet MSL. You do not multiply 1,500 by 100.
Tick marks around the airport symbol indicate that fuel or services are available. They do not change the tower color or the runway length. Anything the block cannot fit, including part-time tower hours, lighting limitations, and the full runway layout, is in the Chart Supplement.
A two-airport comparison
One symbol on the chart is blue and another is magenta. The blue airport has a control tower. If a star is printed with it, the tower is part time and you check the hours. The magenta airport has no tower. You self-announce on the frequency marked with the circled C. If the magenta block's runway figure is 40, the longest runway is 4,000 feet, and the elevation number beside the lighting code is feet MSL. Neither color tells you the tower is staffed right now, because the magenta airport has no tower to staff and the blue airport's star, when present, sends you to the supplement.
The trap
The color questions are missed by calling blue a long runway, a seaplane base, or proof the tower is open. The runway question is missed by reading 40 as 40 feet or 40,000 feet. The elevation is missed by treating it as AGL, as a frequency, or as a number you scale in hundreds of feet the way you scale the runway. Blue means a tower. Magenta means no tower. A star means part time. 40 means 4,000 feet. Elevation is MSL, and it is the elevation figure on the data-block line, not the CTAF and not the AWOS.
Practice a data block on the free practice test. Airport reading is in the course.