Spot colour printing works best for tshirts that don't require a photographic image - basically where each colour in the design can be easily identified. A specific range of colours called pantones are used for this type of printing. Pantone colours are a well-established reference used in printing and design and each pantone has a unique name and reference number. This means a printer can apply a specific pantone colour at any period of time and know that the colour will look the same on a tshirt as the first time it was used.
If your designs involve the use of photographic images or illustrations that use a lot of colours and different tones then this printing method is best suited for your tees. The 4 colour process uses translucent inks that can blend together on a white background and therefore reproduce all the colours used in your design. This method of printing is quite difficult to process on fabric and is only suitable for white tshirts. This also means that costs are higher as it takes more to set up and you'll generally find that printers like to do longer print runs with this method to keep costs down.
Again if your design is more intricate using lots of colours and blends but you need it printing on a coloured tee then a method called 'simulated process' is likely to be used. Like spot colour the design is separated into various colours and shades to recreate the overall look of the original design. Be warned this is more expensive than the previous two methods mentioned and is only for larger print runs.
This type of printing is generally low cost and most suitable for use as gifts, hen night parties, or promotional tees. Tshirt designs printed this way are usually durable and wash proof but won't last as long as the traditional printing methods.
Plastisol ink is the most common ink used for printing on tshirts and works well on all coloured tees as well as white ones, and is generally very durable. Water Based inks are used for white tshirts giving a soft feel though they will fade after a few washes. A variety of special effects can also be applied using Plastisol or Water Based inks such as metallic or glitter effects. Check with a printer to see what effects they can apply to your design as new techniques are always being created.