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We analyze rapid-cadence, multiwavelength photometry of AR Scorpii from threeobservatories, covering five observing seasons. We measure the arrival times ofthe system's beat pulses and use them to compute an updated ephemeris. Thewhite dwarf spin-down rate is estimated with an uncertainty of only 4%. Theseresults confirm, beyond any doubt, that the white dwarf's spin period isincreasing at the rate consistent with by that of Stiller et al. (2018). Westudy the evolution of the beat pulse's color index across the orbit. The colorof the primary pulse maxima varies significantly across the orbit, with thepeaks being bluer after superior conjunction than in the first half of theorbit. Specifically, at orbital phase 0.5, the color index of the primary pulseshows a very sharp discontinuity towards bluer indices. This supports thePotter & Buckley (2018b) synchrotron emission model where the two emittingpoles differ significantly in color. However, no corresponding jump in thecolor of the secondary pulses is seen. Furthermore, our analysis reveals thatthe arrival times of the pulses can differ by as much as 6s in simultaneous $u$and $r$ photometry, depending on the binary orbital phase. If left uncorrected,this wavelength-dependent timing offset could lead to erroneous measurements ofthe spin-period derivative, particularly with heterogeneous datasets.

An Improved Spin-Down Rate for the Proposed White-Dwarf Pulsar AR Scorpii