The unloaded PCB appears green because thin sheets of green plastic have been applied to both sides (otherwise the PCB would appear pale yellow). Called solder masks, these sheets cover all exposed metal other than the component pads and holes so that errant solder can not inadvertently short (or electrically connect) the printed wires. All metal surfaces other than the exposed pads and holes (i.e., the wires) are underneath the solder mask. Not infrequently, blue or even red solder masks are used.
Variable resistors and potentiometers each augment the standard resistor symbol with an arrow. The variable resistor remains a two-terminal device, so the arrow is just laid diagonally across the middle. A potentiometer is a three-terminal device, so the arrow becomes the third terminal (the wiper).
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