<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> An advanced dense wavelength division multiplexing small form factor pluggable (DWDM-SFP) transceiver for the <emphasis emphasistype="italic"><formula formulatype="inline"><tex Notation="TeX">$L$</tex></formula></emphasis>-band, which features a wavelength tunability of 3 nm, is proposed and developed. By using the developed wavelength-tunable DWDM-SFP transceiver, a colorless optical network unit (ONU) prototype for coexistence-type wavelength division multiplexing passive optical networks (WDM-PONs) that enable coexistence with existing PONs is achieved. Both wavelength and temperature dependencies of the colorless ONU prototype are evaluated and the applicability to the existing Class B+ optical distribution network that has a 28 dB loss is discussed. </para>