Color can be used to indicate mood and state of mind, both of which religions hope to influence. Some Earth religions believe that achieving an enlightened state is expressed using a color for wardrobe, buildings, and decorations. This could mean that someone is expected to wear those colors at ceremonies or significant events like marriage or a life milestone. Priests of a lower rank might be denied a color like white, which is the most enlightened state because it represents all colors; a rainbow can carry the same significance.
We can put any plausible spin on a color, making it seem good or bad. For example, red is often associated with passion, sensuality, and blood. Another association would be purity (Hinduism). Most of these are good, but while blood keeps us alive, we tend to ignore this unless it’s spilled, which is bad. Yellow can be associated with fire, which is good unless it’s out of control, but others associate it with happiness. Blue is considered cool and soothing, but some religions find it brave and manly instead. Make it believable and our decision is taken as truth for our inhabitants.
All religions have at least one revered symbol. To create this, use an attribute of the god, a prophet, or a story involving them, their behavior, or the actions of the most prominent followers from the religion’s earliest days. The average person, not a skilled artist, should be capable of drawing this symbol, which needs to be simple.
Obvious examples of attributes include a depiction of the sun for the sun goddess, a lightning bolt for a storm god, or a skull for the deity of death. We can be more creative, but expected icons are powerful and easy to remember. Practitioners want strength, clarity, and confidence, not confusion, wondering, and trying to decipher meaning. As world builders, we have other opportunities to be creative. If the god uses an item, like Thor’s hammer or Poseidon’s trident, these are easy choices, but a god might lead multiple religions, so if one is already using a symbol like Thor’s hammer, another is less likely to do so, or at least with a twist on it.