Bright fibres make for beautiful images, but they are actually the dark side of fibre optics! Optical fibres are intended to transmit light from one end to the other, without the light being lost on the way. In the image, light is being scattered out of the fibre by the glass, and so the bright fibre comes at the expense of less light reaching the far end.
The fibre shown is transmitting white light, containing all colours, and within Proteus such broadband performance is essential. However, the light scattering by the material is significant at blue and ultraviolet wavelengths, leading to a blue glow coming from the fibre. However, much less light is lost at the green and red wavelengths which are most important to Proteus. The origin of the blue colour is thus closely related to the blue of the sky.