Period-(average) effective temperature relations for fundamental mode Cepheids based on models with average rotation (ω = 0.5) and different metallicities. We show predictions at the blue and red IS edges in same colors, IS-averaged relations in yellow, and averages over crossing and IS position in black. Dashed lines represent second crossings, solid lines third crossings. We also show spectroscopically-determined T_eff of Galactic Cepheids (left panel, where red, yellow, and green errorbars correspond to data from Luck & Andrievsky 2004; Kovtyukh et al. 2005; Andrievsky et al. 2005). We plot their phase-averaged values as larger symbols with standard errors and the individual (phase-resolved) measurements as smaller circles of the same color. Average effective temperatures inferred by global modeling for Galactic, LMC and SMC Cepheids (Pejcha & Kochanek 2012) are shown as small black error bars. See text in Sect. 3.6 for a discussion of the mismatch at lower metallicities.