On a 1 to 100 chart with 10 numbers in each row, some squares are shaded orange. Which rule decides exactly which squares are shaded?

The shaded squares are 6, 12, 18, 24, … — every 6th number. These are even AND have a digit sum that is a multiple of 3. Multiples of 3 alone would include 3 and 9; even numbers alone would include 2 and 4; multiples of 12 would leave out 6 and 18.