Visible Under Ultraviolet Light
It is possible to see the Black serial numbers with the naked eyes, but under ultra violet light, the numbers fluoresces green.

Similarly, The 1000 naira banknote paper has florescence scattered fibers which appear red and blue under ultra violet light (UV Light). The back of the note shows two large numerals 1000 as well as a small circle near the top of the note when viewed under UV Light. The two solid segments in the circle are blue under normal light but turn to light blue and green when viewed under UV Light. The white section of small circle also turns violet UV Light.