Null does not burn, melt Snow, evaporate, or damage Elastic or String. This makes it useful when you need a liquid that does not interact with other important parts of your creation.
There are two keys for Null: A and N. When you select the material with those keys (and not pressing escape) the two materials will have the same color. When you use the Mix Mode by pressing esc + A + esc to make A-type Null, it is paler than normal.
Null does not have the "surface water" effect that Water does, which could be aesthetically useful.
Save-file analysis has revealed why A-type Null and N-type null have different colors. A-type null represents the "surface material" of a liquid, the paler particles that occur near the top of Water. A-type Null thus represents "surface water" and N-type null represents "deep water". This is why the game considers them to have the same base color but renders A-type null a paler color.
A-type Null and N-type Null can be mixed with Snow to produce a new hidden material. The material AY makes a paint-like material, the material NY is normal snow.