In Hudson's presentation of JUnit test results, the colours for failed and skipped tests are switched in the test count graph: failed tests are yellow, and skipped tests are red. This doesn't happen for the test duration graph (the default view).
To reproduce, open "Test Result" for a build, open test history by clicking the "Took X seconds" link, and switch to count view through the "show count" link. Now compare the graph to the table below: it turns out that failed tests are displayed as yellow, and skipped tests as red.
In the attached graph, tests are regularly skipped (shown as red), but the table shows that there has been no failure since build 1347 (shown as yellow).