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Not sure what this has to do with Leopard"...
It may depend on the version, but Excel in Ofiice 2004 takes its highlight colour from the user's global setting which can be changed in "System Preferences" > "Appearance" > "Highlight Color:". Use "Other…" to select arbitrary colours beyond the pale ones listed by default. That would make this an "OS X" question, since "Excel" doesn't appear to have its own control for changing the colour.
Note that Excel has to be relaunched for the setting to take effect, and unfortunately, the '
AppleHighlightColor' key doesn't seem to work in Excel as a per-app setting like it does in other apps.