Normal sudoku rules apply. For each set of 3 lines that are the same colour (and line styling), two of those are index lines and one is a thermometer. No line can be ambiguously both. Digits along a thermometer must strictly increase starting from the bulb. The digit in the Nth cell along an index line (starting from the bulb) indicates the position along the line where the digit N appears. For example, 3214 is a valid index line, the 1st digit, 3, indicates that the 3rd digit is a 1, and so on.