I have for a long time looking in a way to darken the fretboard on my RG2820, it was very light. Now finally after reading in different forums I have find out how to do. The result almost looks like an ebony board, and it looks far much better then what the pictures can show.
This is what i used:
Fiebing's Professional Dying Oil
How did I do?
I started to mask all areas that not should have the oil on it, mask very carefully, the wood sucks in the oil very quickly, and I don't think you can remove it.
Then I used cotton pins to apply the oil, I applied the oil to 4 frets at each time, then I waited for one or two minutes for the oil to sink in. After that I used a rag to rub it off, rub until the rag not will be colored any longer. Repeat until you have done the entire fretboard.
Then i removed the mask and applied lemon oil, I'm very happy with the result.
I went out and took a some new pictures, those show the result even better!