Every chart is a set of encodings: decisions about which visual property shows which column.
Position on a shared axis is the most accurate encoding, which is why bar charts and scatter plots read so well.
Length comes next, then angle and area, with shades of colour among the least accurate for exact values.
This ranking explains why pie charts, which use angle, are harder to read than bars, which use length.
Colour works best for telling groups apart or highlighting, rather than for showing exact amounts.
Hue means the colour family, such as red or blue, and suits categories because hues have no natural order.
Lightness has a natural order, so ordered values suit a single hue running from light to dark.
A diverging palette joins 2 sequential palettes at a meaningful middle, such as 0 change or the average.
Rainbow palettes are a poor choice for ordered values, because their bright bands create edges that are not in the data.
Categorical palettes stop working beyond about 8 colours, since readers can no longer tell them apart.
Red against green fails many people with colour vision deficiency, so blue against orange is a safer pair.
Checking a chart in greyscale shows whether it still works without colour.
Consistent colours across charts help readers, so one region should keep one colour throughout a report.
In practice a chart often uses grey for context and a single strong colour for the point being made.
The usual mistake is using colour for decoration, which competes with the colours that carry meaning.