Here's what you do...

1. Add a report variable called rnInvCount, set the value to zero, set the Calculate option to SUM, set the Reset point to End of report.

2. In the OnExit Expression of the group footer band for the invoice, put the following:

_VFP.SetVar('rnInvCount', rnInvCount+1)

3. Use the logic of MOD(rnInvCount,2) = 0 (or the opposite) in your shading to determine whether to print the colored background.