A three-dimensional Monte Carlo Lyα radiative transfer (RT) code, named LaRT, is developed to study the Lyα RT and the Wouthuysen？Field (WF) effect. Using the code, we calculate the line profile of Lyα radiation within the multiphase interstellar medium (ISM), with a particular emphasis on gas at low densities. We show that the WF effect is in action: the central portion of the line profile tends to approach a small slice of the Planck function with a color temperature equal to the kinetic temperature of the gas, even in a system with an optical thickness as low as τ0？？？100？500. We also investigate the effects of the turbulent motion of the ISM on the emergent Lyα spectrum and color temperature. The turbulent motion broadens the emergent spectrum, as generally expected, but the color temperature is not affected by the turbulent motion in typical astrophysical environments. We utilize two multiphase ISM models, appropriate for the vicinity of the Sun, to calculate the 21 cm spin temperature of neutral hydrogen, including excitation via the Lyα resonant scattering. The first ISM model is a simple clumpy model, while the second is a self-consistent magnetohydrodynamics simulation model using the TIGRESS framework. Lyα photons originating from both H II regions and the collisionally cooling gas are taken into account. We find that the Lyα radiation field is, in general, likely to be strong enough to bring the 21 cm spin temperature of the warm neutral medium close to the kinetic temperature. The escape fraction of Lyα in our ISM models is estimated to be ？7%？20%.