I bought some warm white gu10 LED spotlights from ebay for just over 2 dollars each. Although warm white I wasn't satisfied with the light colour, it was a bit to blue ('cold') to my liking. Therefore I decided to try and change the colour a bit by colouring the LEDs. When comparing to an incandescent light I still favour those, but when it comes to saving energy, our planet and money the LEDs still win.
NOTE: This might change the shape of the beam a bit, and this certainly will decrease the light output.
With thanks to Lin Que for lending me her fineliner.
Step 1: Dissassemble
First of all, take apart the bulb so you can reach the LEDs. This may vary per type of bulb, or may even be impossible without breaking it. With this particular type it was simply unscrewing the top lid. Make sure not to do this when the light is plugged in (especially when turned on, but also when turned off).
Step 2: Colour the LED
Take something like a marker or a fine liner to colour the LED. I think the fine liner I used is water soluble, but you might even try to use permanent marker. As you can see in the picture I didn't manage to fully cover the LED with a thick layer of colouring agent. This might actually be a good thing, since this allows some of the other colours in the spectrum to still go trough, making the light appear to be white. I did notice some a decrease in warm colour when the light was on for some time after colouring, so perhaps multiple layers are still needed. The ink formed dots when fully dried so the warmth of the turned on LED was probably the cause of this.
Step 3: Reassemble and Power It Up!
The pictures are an example of the original and the coloured LEDs. You can already see a difference, although I did add a bit more after the photo was taken to make the effect a bit better then shown in the picture.