Gradient

I think there are tutorials about this thing as well, but I was asked this a few times so yeah. ;3;/ This is some simple example.

You don't really need gradient tool to change the colour, just as long as you add the overlay layer it will work when you put colour on it.

or as =Silver-MoonNight stated, "Any program that has Layers feature can do this"

If you use paint bucket you'll only get one colour instead of gradient.

For SAI or other program you have to make the gradient manually I guess. ;v;/ You can use the brush tool to colour the overlay layer.

... and sorry for my English. OTL

Put to scrap later.

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Edit: Forgot to mention, if you're dealing with a really dark image, try to add a new (white) layer under the image layer and adjust the opacity of the overlay layer and image layer. Hope that works.

If the image appears too light, you can just adjust the 'level' for the image layer to make it darker/clearer.