Colour palette creation

So there is this magazine that you have got to layout or the webpage which is longing for a new design. But how to you keep colour consistent? Not all colors match against each other.

For this purpose you could create an image in that just serves as canvas for color picking.

So lets start with some random base color. Open up an image(not to big 640x480 should do it) and fill the layer with your desired colour your layout shall be based upon:

Next we add a new layer and fill it with a black/white gradient from left to right and set its mode to "Multiply". Create a horizontal guide to ease the creation of a straight gradient:

This way we already got the colours variations from normal to dark. Let's proceed with the normal to white variations. To get there we duplicate the layer containing the gradient, set its mode back to "Normal" and remove its black part via "Layer" -> "Transparency" -> "Color to Alpha" and the select pure black as the colour to be removed:

But we not only want variations of the colour but also of suplement colour. Therefore we duplicate the layer that has its mode set to "Multiply" and set it to "Difference":

This yields in three layers that can be combined freely to achieve many many matching colours when getting pickes with the color picker. Just ensure that you check "Sample merged" in the picker's tool-options.

This "rigg" could be extended by adding a gradient that slowly fades to grey from the base-colour as an example.

Adjustable old film effect

We will be creating an effect that tries to fake some kind of darkening on the edge of an old film-roll and some noise.

So let's make a start with the following image:

We duplicate the layer and apply the Gaussian Blur Filter as follows:

Now we create a new white layer and apply a radial gradient to it right into the middle of the layer:

To get the right position for your gradient you should use some generated guides. Image -> Guides -> New Guide (by percent). Create two of them at 50%. One vertical and horizontal each.
Now scale the layer in width to about 160% of its size:

Apply the Curves-Tool and a Gaussian blur as follows:

Add a layer mask to our blured layer and copy the layer containing our modified gradient into it:

To get the darkening on the edges we select the whole image via Ctrl + A, select the mask, copy its content via Ctrl + C and paste it back into the image and create a new layer from the floating selection. Invert this new layer and set its mode to "Multiply".

The final step is to add some noise to the darkening layer. Therefore we will be resizing it to 50% of its size, add some HSV-Noise (Filters -> Noise -> HSV Noise):

Now scale it back to its original size but with "Interpolation" set to "None". This way the noise wont get blured and it will stay as crisp as possible.
If you like you still can add some sharpening filter to the layer with the noise.
The cool thing about this setup is that you still can choose the amount of blur and darknes by just modifying the layers' "Opacity"