Color's representation as a series of 4 floats is inconsistent

In some cases (like Gizmos) this is stored as an `[f32; 4]`. In other places, like `StandardMaterialUniform`, this is a `Vec4`. I think a `[f32;4]` is slightly more clear, but I really don't care which one we standardize on.

Start by locating the color representations used by Gizmos and StandardMaterialUniform, then trace other four-float color representations across the relevant rendering code. Decide which representation should be standardized and update the affected uses consistently; done means the inconsistent forms have been resolved and the relevant checks still pass.

Written by the indexing model from the issue text.