We used optical confocal microscopy to study optical properties of diamond 50 nm nanocrystals first irradiated with an electron beam, then dispersed as a colloidal solution and finally deposited on a silica slide. At room temperature, under CW laser excitation at a wavelength of 514.5 nm we observed perfectly photostable single nitrogen-vacancy (NV) colour defects embedded in the nanocrystals. From the zero-phonon line around 575 nm in the spectrum of emitted light, we infer a neutral NV0 type of defect. Such nanoparticle with intrinsic fluorescence are highly promising for applications in biology where long-term emitting fluorescent bio-compatible nanoprobes are still missing.