Here's why:

* Gaseous SO₃ is colorless: In its gaseous state, sulfur trioxide is colorless.

* Liquid SO₃ is colorless: In its liquid form, sulfur trioxide is also colorless.

* Solid SO₃ can appear white to slightly yellowish: However, solid sulfur trioxide can appear white or slightly yellowish due to impurities or the formation of other sulfur oxides.

Important Note: It's crucial to avoid confusing sulfur trioxide (SO₃) with other sulfur-containing compounds, which can have different colors. For example, sulfur dioxide (SO₂) is a colorless gas, but when dissolved in water, it forms sulfurous acid, which can be yellowish.

So, while sulfur trioxide might appear slightly yellowish in its solid form, its primary color is colorless.