`lc variable` tells the program to use the value read from one column of the input data as a linetype index, and use the color belonging to that linetype. This requires a corresponding additional column in the using specifier. Text colors can be set similarly using `tc variable`.
Examples:
# Use the third column of data to assign colors to individual points
plot 'data' using 1:2:3 with points lc variable
# A single data file may contain multiple sets of data, separated by two
# blank lines. Each data set is assigned as index value (see index)
# that can be retrieved via the using specifier `column(-2)`.
# See `pseudocolumns`. This example uses to value in column -2 to
# draw each data set in a different line color.
plot 'data' using 1:2:(column(-2)) with lines lc variable