Nanofluids of selective visible colors are developed in gold nanoparticles embedded in polymer molecules of polyvinyl pyrrolidone (PVP) in water. A simple HAuCl4 reaction with PVP molecules (a weak reductant) in aqueous solution at 60-70 degrees C yields the nanofluid in a single process step with an AU3+ --> Au caloric reduction. Part of PVP molecules, which binds to a resulting Au-nanoparticle of nascent surfaces, insulates it from the Au3+ species and thus disrupts it to grow further, by immobilizing in the PVP molecules. Varying the 0-1.0 wt% Au-contents (in Au-PVP) tunes effectively the Au-surface plasmon bands (SPR) in Au-core-shell particles in conjunction with the surface PVP molecules. Characteristic SPR absorption band shifts regularly from at 535 nm in 0.05 wt% Au to a value as large as 571 nm upon increasing the Au-content to 1.0 wt%. The SPR band favors emission at approximately 440 nm in a core --> shell energy transfer in core-shell particles of an exchange coupled system, when irradiating at approximately 250 nm in the 5d(10)6s --> 5d(9)6sp (Au) interband transition. Electron microscopic images, with the electron diffraction, reveal small Au-crystallites in shapes of cuboids (or bars) of core-shells, with 5-10 nm width (5-15 length), and their clusters of mostly pentagons.