Grey exclamation marks are used for emphasis or degree. For example must and important.
The exclamation mark used for emphasis is also used for comparatives and superlatives. One exclamation mark is used for the comparative and two for the superlative to show the increased emphasis.
There are a number of common negatives which are normally indicated with a red line, in both colour and black & white symbols. Other negatives are created with the use negative symbols: