Conditional Format Based On Previous And Current Record

I'm having trouble with a tricky conditional format in a continuous subform.

The criteria is based on field [GroupNo] when it changes (increases) which are always listed in numerical order.

Basically all I want to is for the text box colour to alterante (eg blue & yellow)when GroupNo changes value

So it would look something like this....

Group NoID (highlight colour)

1 (yellow)
1 (yellow)
1 (yellow)
2 (blue)
2 (blue)
2 (blue)
3 (yellow)
3 (yellow)
etc etc

btw, although values will always be in order they may not all be output, so it's not just case of odd, even, odd, even (that would be too easy!)

I can retrieve the Previous Record's GroupNo value (via a function) in another text box and compare current GroupNoID with this in the Format Expressions and manage to change the colour, but it won't sustain this new colour for the subsequent same GroupNos, obviously as the values have changed.

So how can I....

Change the colour when the value changes.
Keep this new colour until it the value changes (increases) again.

Many Thanks