Cationic conjugated polyelectrolytes (CPEs) containing an iridium complex ((ppy)(2)Ir(FlPy)) as the energy acceptor were synthesized via Suzuki coupling reaction. The polymer with 12 mol % (ppy)(2)Ir(FlPy) (P4) shows efficient intrinsic fluorescence resonance energy transfer (FRET) in aqueous solution due to the solubility-limitation-caused polymer aggregation. Thus, at a relatively high concentration, P4 emits dual-emissive red phosphorescence upon donor excitation. Investigation of the emission responses of P4 toward proteins reveals a unique weakened FRET process from the main segments (donor) to the (ppy)(2)Ir(FlPy) units (acceptor) in the presence of histone instead of other proteins. With such a selective weakened FRET process, the emission color of P4 solution turns from red into lilac only in the presence of histone, allowing for visual discrimination of histone. Moreover, the linearity of this weakened FRET of P4 also enables effective quantification of histone with a limit of detection of 0.06 mu M. This study thus provides a new design concept to take full advantage of polymer solubility limitation for CPE-based multicolor visual sensing.