Situation: The floss supplied in a kit is of poor quality.
If you are lucky, the chart supplied with the kit lists colour numbers and a brand name. This doesn't happen very often, at least with kits that supply ugly floss. If there is no list, try to get a colour card for one of the big-name brands of floss such as DMC or Anchor. Look for one which includes thread samples. Match the colours from the kit with the colours on the card as carefully as you can. Do it in natural light. Write down the numbers of the colours you need on the chart next to the correct symbol. If you can't find a colour card, take the bad floss with you to your local needlework store and do the matching there. Be careful, because the lighting in some stores can make the colours look wrong.
Situation: You created the chart yourself.
If you are experienced enough to create your own chart, you are probably experienced enough to select fibres. Consider using the many new types of fibres which are now available, such as metallics and hand painted silks. Always keep in mind the final use of whatever you are stitching. For example, don't use a non-colourfast silk for a baby's bib.
Situation: You want to use a different brand of floss than suggested.
Some charts supply colour number information for two or three manufacturers' floss. If not, try to find a floss conversion chart. Commercial ones are available and there are conversion charts in the "Needlework FAQ: Threads, Fibres, Embellishments".
Situation: You want a different texture or finish.
Consider using the many new types of fibres which are now available, such as metallics and hand painted silks. Always keep in mind the final use of whatever you are stitching. For example, don't use a non-colourfast silk for a baby's bib.
Situation: You want to use different colours than suggested.
If it is a geometric design or a simple picture with no shading, replace the colours anyway you like. More care must be taken for complex pictures. Compare the values of the old set of colours and the new set to make sure they are the same. You can do this by looking at the threads through red glass or cellophane, or by photocopying them in black-and-white.
While we're on the topic of fibres, here is a definition, just in case you ever see references to "Z-twist" or "S-twist."
From: Noeline McCaughan
Just to make things a little clearer -"Z" and "S" are used to describe the twist in a yarn - any yarn regardless of what fibre it is spun from. Just take a piece of thick yarn and hold it up in front of your eyes. If the twist goes from top right to bottom left it is called "Z" (the slant of the twist equalling the slant of the downstroke in the letter). If it slopes from top left to right bottom it is of course an "S".
source : www.mismatch.co.uk/