as you mention, "I was hoping that they would have to work at this, knowing
that when they got the formula exact, the cell would change colour, and so
they would then realise that they had got the formula correct."

I suggest that its up to you to place the conditional format, on their
workbook for checking purpose, when they are completed and submitted to you,

You can ask the students that after they had made the times table, say in
sheet1, they will create another table say,say in sheet2, wherein the
formulas are in text,

This can be done by [Find = then Replace with .= _ ]

from the sheet2 text table, you can create the conditional format. But you
may not share it to the students, otherwise- it will be a spoonfeed.