In this paper, we propose a two-tiered colorless WDM-OFDMA PON system architecture that draws strengths from each individual WDM and OFDM PON systems. Specifically, the two-tiered architecture enables a colorless transceiver to be shared by a group of ONUs, resulting in drastic reduction of the system cost. For achieving colorlessness via reusing downstream light sources, we discover the residual power of downstream signal unexpectedly springs back after transmissions, causing severe interference to the upstream signal, and thus limiting the data rate of the upstream signal. We devise a method of adopting a common dispersion compensation module at OLT to reduce the residual power over all wavelengths. Experimental results show that, with an improvement of upstream signal's SNR up to 10dB, the system successfully achieves 20- Gb/s bidirectional OFDM-signal transmissions on the same wavelength over a 20-km SMF.