Niobium is a reactive metal like titanium, which is easily anodized using voltages from 10 to 140V.DC in an electrolyte. The electrolyte of choice is trisodium phosphate 3% in deionised water. The colours which appear vary according to voltage, from bright green at the high end, through purple, red, orange, various blues and finally brown and dark grey. I found that at high voltages the colours developed too quickly and became muddy. To counter this I halved the strength of the electrolyte by adding more water.
The layers formed are thin and not wear resistant, but might be useful in recessed areas and as backgrounds to pierced work. The dedicated apparatus from Reactive Metals costs $250, plus freight, taxes and other charges, which would almost double that. Having read a brief account of a method using batteries, I decided on a pilot study along that route, but using brush anodising to make a test strip.
The power source is 14x 9V. Batteries, connected in series. The anode is the test strip. The cathode is a synthetic artistís brush, with a wire soldered to the ferrule and insulated with tape. The brush is moistened sparingly with the electrolyte. Voltage is selected by attaching the insulated crocodile clip to the various positive terminals in turn.
Apart from the batteries, which cost £1 each, the recycled electrical flex and the crocodile clip, all that was needed was equipment for soldering. The other bits and pieces were only added out of an obsession with tidiness. I did also add a switch to avoid an accidental short circuit. For more control at the high voltages it might be useful to insert a variable resistance, rather than dilute the electrolyte.
Should you want to do this yourself, please work under the supervision of a qualified electrician and wear rubber gloves. The batteries may look innocuous, but linked together can give you a serious shock. Dennis.