Turns out that in van der Poel's case at Flanders there was a clear strategic objective. For race team support it wasn't possible to quickly distinguish van der Poel from Jungels. Their respective national jerseys are only slightly different in tone of blue (Luxembourg's is slightly paler than NL). With the shorts they were able to identify him easily in live footage. Of course they may have brought some bad luck with the planter incident so now its back to black.