What is Colour Separating for T-shirt Printing

What is colour separating for T shirt printing?

Printing onto T-shirts using the traditional screen printing process is not always straightforward.

There are variables, which will determine the approach.

The first variable is the image itself and how it will be colour separated before being sent to the screen room.

Artwork is always constructed using some basic but variable building blocks that require us consider how they will be screenprinted;

  • Is it a vector or a bitmap file format?
  • Is the resolution of the artwork good enough to begin with?
  • Artwork will contain one or more colours. How many is too many?
  • The ‘object’ in the artwork may be solid spot colours or they may contain gradients and tonal changes.
  • It may contain text. The size of this is important.
  • The artwork may contain key-lines and their thickness is important.
  • What colour garments are we printing on?

Many customers will not consider that so much is required to assess artwork. If they are lucky enough to have had a graphic designer produce their artwork then much of this thinking will already have occurred if the graphic designer is ‘t shirt’ trained.

Screen printers are often supplied with low-resolution images from websites for reproduction. This is rarely adequate for direct reproduction as web graphics are designed to be as small a file size as possible so that the web page they are lifted from has a faster loading speed.

These images often have to be re-drawn using a vector program such as Adobe Illustrator or they are put through a number of Photoshop filters to try to improve them. Sometimes both.

Assuming the artwork is the best it can be, there are a number of colour separation techniques that we use.


This is the basic and most often used process. It is used when your artwork is comprised of solid colours that do not contain complex photo realistic tonal variations or colour variations known as gradients. These can be done using Adobe Photoshop or Illustrator and the idea is that each colour is lifted from the design, placed in its own layer within the layer pallet of the Adobe programs, converted to a black infill and then labelled with the colour information and registration marks before being printed onto vellum or acetate film for the screen room. Each colour fits snugly next to the other colours with no overlap.




    This type of printing uses 4 colours to produce multi-coloured designs containing hundreds of colours. Process printing is often described as CMYK printing with the letters standing for CYAN, MAGENTA, YELLOW and KEY (which is black). All magazine pictures are printed using this process and if you hold a magnifying glass up to these pictures to will see a variable pattern of small dots.



    The resolutions achieved with modern digital paper printing presses is much greater than those that are able to be achieved with screen printing and these dots would be difficult to see with the human eye in a magazine but would be very evident on a T shirt.This is because the output resolution to the film required to burn a screen cannot be any higher than 65 lpi or dpi.

    This is down to the restrictive capabilities of the chemicals and processes used in screenmaking for screen printing.

    Process separations can work well on white T shirts but are not so good for dark T shirts. This is because the process inks used are slightly transparent and the colour of a dark shirt simply shows through the printed ink and affects print colour. It is possible to print a white base under process inks but this often results in a ‘washed out’ colour effect that prevents the final image from looking as was intended. It is common to augment process printing with spot colour printed in a halftone to fit into the dot configuration of the process separations. It is never possible to get Pantone accurate colours with process printing and much of the skill in producing a good print lies with the printer who is setting the job up on the press in conjunction with good separations provided by the artwork department.

    These separations are printed on high mesh count screens that are designed to hold detail. Mesh counts of 90 holes per inch up to 140 holes per inch are typically used.



    This process uses spot colours output to halftones similar to how the CMYK halftones are rendered.

    This is considered a better option than CMYK separations for complex, tonally rich T shirt printed graphics as it produces bright, punchy colours regardless of the T shirt colour. A white underbase is used and this requires great skill and experience to understand and separate correctly. Photorealistic images can be achieved with this process which can sometimes be called ‘channel’ or ‘tonal’ separations.

    These separations are printed on high mesh count screens that are designed to hold detail. Mesh counts of 90 holes per inch up to 140 holes per inch are typically used.

    simulated-printing process


    Sometimes called DIFFUSION, DITHER OR STOCHASTIC, this system uses small randomly spaced squares rather than dots or ellipses to create the tonal variations.


    The squares are printed next to each other rather than overlapping like process or simulated process printing. This creates a ‘jigsaw’ effect which creates the tonal variations.


    Observes closely, you can see the image takes on a ‘posterised’ look but as you move away, the human eye cannot pick up this effect and the image looks tonal and variable.

    When to use what?

    Most of the time spot colour separations are used as most artwork seems to be simple text or solid coloured images.

    Where a tonally rich and variable design is required then the ‘go-to’ system tends to be SIMULATED PROCESS PRINTING.

    The only time this process might be altered to another is if there are cost considerations from the customer preventing this system being used. If the budget only allows for 4 colours to be used and if we are going on to white T shirts then CMYK might be the option.

    If the design is tonally variable but a ‘high contrast’ effect is required then maybe INDEX would be the better solution.

    Your  sales advisor is trained and supported by 25 yrs of experience to be able to get you the best possible solution.

    Author Arron Harnden


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