Screen display goes weird when launching an application

I've been using the excellent Mail Washer program to
filter spam for several months but just recently,
whenever I open Mail Washer, the screen goes black for a
couple of seconds then comes back on but with half the
colours missing or different. Everything still works OK
but it's making me nuts.

I've tried reinstalling/redownloading Mail Washer and
checked my drive for errors (none). Any suggestions?

Screen display goes weird when launching an application