This is the result of an attempt to do with only dice something analogous to the Universal Table from TSR's Marvel Super Heroes RPG.

A skill test mechanic

Roll three 20-sided dice, aiming to roll less than or equal to a target number. This value will usually be the character's skill level plus or minus any modifiers. If one die succeeds, then we call the result "green"; if two succeed, the result is "yellow"; if three succeed, the result is "red"; and if none succeed, the result is "white".

A green result or better is usually considered a success, while a white result will generally indicate a failed attempt. Green, yellow, and red results may indicate different magnitudes of success, or specific game outcomes, depending on the circumstances.

The table below shows the probabilities of the various results for target numbers from 1 to 20.

For exact probabilities: AnyDice

The effects of modifiers at different levels of skill

Consider two characters, a novice and an expert, each using a rifle in an assassination attempt. A green result is sufficient for a character simply to hit his target, while a yellow result indicates a critical injury, and a red result means that the target is killed instantly.

Now suppose that circumstances warrant a +1 bonus to the characters' skills. For the novice, with a low skill level, this bonus will correspond to a significant increase in his basic chance to hit. Meanwhile the expert, with a high level of skill, would find that his chance of getting a green result doesn't increase by very much at all; by virtue of his higher skill, his ability to hit is virtually assured, and the small bonus doesn't mean much to him. However, the expert will find that the +1 bonus gives a large increase in his likelihood of getting a red result for an instant kill. So a bonus helps a novice to succeed, but helps an expert to succeed well.

This contrasts with a simple diminishing-returns system like GURPS, where any modifiers just have lesser value at higher levels.