Looking specifically at creating spot colour positives.
This is the way you usually work with screen printing, the CMYK method is not as relevant to the everyday screen printing method.
How to access the Pantone colour reference system in Photoshop.
Click on foreground colour because this is the active colour.
Click on colour libraries to view the spot colour libraries.
PANTONE solid uncoated.
You can type numbers to find a specific colour swatch you are searching for.
Using Pantone colour in Photoshop and then printing it with a CMYK printer will mean that the colour will be mixed using CMYK, not using a spot ink. Therefore it will not look the same colour as the Pantone swatch.
When using spot colours in Photoshop, we need to work in a different way to what we usually do.
Channels
If we have channels for cyan, magenta, yellow and black then it is possible to have a channel specifically for a spot colour, so that it will be printed with its own ink.
Everything is going to happen using channels instead of layers.
A separate channel needs to be made for each colour.
Channels -> Menu -> New spot channel
Click on the colour square to select a colour.
For digital print you could use the Pantone system.
For screen print, the pantone system cannot be perfectly mixed with the inks in there so just use a colour close to what you are going to use so that it will help you visualise the image you will be producing.
Make sure the layer is named so you can differentiate which colour it is.
CMYK channels are in greyscale.
So if we are using our spot colour channel we can use black and white.
D = shortcut to make black our foreground colour.
Paint with black to act like you are painting with colour.
Paint with white to act as an eraser.
When you make the grey channel invisible then you can see the colour you have applied in black and white form. This will be your positive for screen print.
Double click the spot channel to get spot channel options.
You can then change the colour and the whole spot channel will change to that colour.
This allows for quick colour experimentation.
Remember to change the name of the spot channel so the colour label doesn't become confusing.
How to separate the colours of the image using spot colour channels.
Select -> Colour Range -> Select a colour
Selection Preview -> White Matte (so you can view the colour that you have selected.
Adjust fuzziness to select the amount of pixels you want to include.
Create new spot colour channel.
You will then be viewing the spot channel and CMYK channels together which is misleading.
Hide CMYK channels to view spot channel in isolation.
When using channels you can use any photoshop told got clean up the image as you normally would.
Repeat process for each different ink you will be using.
Make sure the CMYK layers are selected when you make the colour selection.
When you have all of the spot channels, including black, you can easily play around with colour combinations.
File -> Save As -> either TIFF or PSD files as these support spot colour files.
Make sure spot colour box it ticked.
Open a blank illustrator file.
Place the image into the document. Ignore what the colours look like at this stage because they are misleading.
Window -> Separations preview -> click overprint preview.
Turn off CMYK and just leave the spot colour channels you have created.
File -> Print -> Black and white laser printer
Output -> Mode -> Separations (host-based).
Turn off CMYK to leave just spot colour channels. This means these are the only ones that will be printed.
There are no tints or halftones so the angles of printing don't matter. We are only working with solid colour.
Thus method is good for…
Editing colour
Experimenting with colour combinations
Being able to quickly go between colour and black and white positives
There is the option to 'split channels' if you need to use the larger scale printers and need the positives as separate files. It is best to save a copy of the original image before splitting the channels.
Working with tints of a spot colour.
Taking advantage of one colour being a lighter tint of another and thinking how we can use one ink to create this instead of two.
Instead of using just black to paint onto the spot colour channel, use a range of greys to get different tints of the same colour.
Create new spot channel for darkest tone (this will appear black).
Then select the lighter using colour range and then while the dark colour spot channel is selected, use a grey brush to paint in this selected area.
The lighter colour must be converted into halftones for it to be able to work in screen print with only one ink.
When printing…
Black is a solid ink so halftones don't need to be considered.
Overprinting and Knocking Out
How to manage these when working with spot colours.
Overprinting is where the colours overlap and merge to make another colour.
The image uses yellow and magenta but in their own spot colour, not using the CMYK channels.
The default transparency is 0%. Changing the solidity accurately shows the transparency of actual printing ink because printing ink is not 100% opaque. This is how you get blending of colours in printing. Changing the solidity to 100% will make it opaque but this is not realistic to how it will be printed. Black is usually an opaque ink.
As you go down the channels palette, this is the order in which inks will be printed.
Changing the order will affect the image if some of the channels are opaque and some are transparent.
This means we can print with two inks but by overprinting, we can create an additional colour during the printing process.
Knocking out is the opposite of overprinting. There will be no mixing of the inks during the printing process because none of the colours actually overlap. But it means that the two colours would have to be very accurately registered to fit into its assigned space.
Because you know black is going to be 100% opaque, you can take advantage of this and allow the colour to extend under the black to minimise any mis-registration.
This is called trapping in commercial print.
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