A chart lists the colour of each symbol by a thread number. The thread may be out of stock, the shade may not suit your room, or you may have a drawer of floss you would like to use. Changing a colour is easy, but the aim is to keep the picture working, and that depends on more than matching a name.
Keep the value
Value is how light or dark a colour is. A chart reads as an image because neighbouring colours differ in value: a light highlight beside a mid-tone and a dark shadow. If you replace the mid-tone of a flower with a colour that is as light as the highlight, the petal flattens and the edge is lost. Always check that the new thread sits at the same place on the scale as the one it replaces.
A simple test is to lay the threads side by side and look at them through half-closed eyes, or take a photo in black and white. Colours that blur into one grey have the same value.
Stay inside the family
The thread colours database groups each colour by family, such as pink, red, green and blue, and lists each by number, name and hex value. Shades within a family are designed to step evenly from light to dark, so a substitution inside the family keeps the gradient. To find a replacement, open the family of the original and choose the neighbour: the next lighter or darker step. Hex values let you compare on screen, but screen colour is only a guide, because floss looks different in daylight, and dyed floss has a sheen that a screen cannot show.
Change whole ranges together
If you change one colour in a gradient, change the others in the same gradient. A chart with five greens for a leaf, shifted from yellow-green to blue-green, looks right when all five move. If only one moves, it becomes a patch. Plan the change on a copy of the chart, writing each new number beside the old one, and check that the new set still steps from light to dark.
Watch the neighbours
A colour changes in the company of others. A grey looks warm next to blue and cool next to orange. If you swap a background from white to black, a pale thread that looked soft now glows and might need a tone down. Lay a few stitches on the ground fabric before committing to a full area.
Use fewer colours
Charts converted from photographs can use 40 or more colours, many of them very close together. Merging near-identical colours makes the stitching easier, with fewer threads to manage, and the piece often looks cleaner. The photo to chart tool in the tools section returns a count for each colour, and lets you see which colours are used in just a few stitches. Colours used for fewer than about 20 stitches can often be merged into a neighbour with little change.
Substituting with another brand
Thread makers number their colours differently, and a conversion chart between two brands is an approximation, not an exact match. Compare the actual skeins, not the printed numbers. If two brands are mixed in one piece, use them in different areas rather than next to each other, since slight sheen differences show along the boundary.
Variegated and overdyed thread
Hand-dyed and variegated flosses change colour along the thread. They produce a mottled look that suits skies, water, foliage and backgrounds, but they look patchy in areas that should be even, such as a face or lettering. They also change the scale of the colour spread depending on how many strands are used. Test a few inches first, and cut the thread in lengths from the skein in the same order so the pattern of colour repeats evenly.
Metallic and specialty thread
Metallic floss adds a sparkle for a star or a border, but it is stiffer, frays and is harder to pull through the fabric. Use shorter lengths, a larger needle, and a single strand, often with a strand of ordinary thread alongside. A plain thread is best for the body of a design, with metallic only for accents.
Count the amount
When you substitute a colour, its stitch count stays the same, but if you merge two colours into one the quantity adds up. The thread estimator gives metres of floss and skeins for a stitch count at a fabric count and strand number, so that you can work out whether one skein is enough. A skein has about 8 metres, enough for roughly 400 to 500 stitches on 14-count aida with two strands, although this varies with tension.
Record what you changed
Write the changes on the chart itself, with the old number crossed out and the new one written beside it. Put the same note on the thread organiser. When you come back to the project after a break, or you want to stitch the chart again, you will know which version you used.