Alison, go to Tools/Options/[Color].  The colours under the thin line
labelled with "Chart fills" and "Chart lines" respectively are the colours
Excel uses when generating a chart, the first with the most left one, etc.  
Simply change them there and when you save yr Excel file the scheme will be
kept for this file.  The scheme can be copied to an empty Excel file which
you can use next time as template.
BTW, if you use the same type of chart over and over, a more efficient
option may be to create yr chart with default colours and settings and add
this as custom-built template.
HTH,
Henk