Flat Silver tends to be much less swirly than Pearl Light Gray. For Pearl Light Gray there seems to be a thin clear layer over the silveryness or swirlyness. Pearl Light Gray tends to be, but may not always be, lighter in grayness. Flat Silver has no such layer but sometimes the normal shinyness of normal LEGO plastic fools people.

LEGO uses different plastics and plastic formulas, and that has an effect on color. For parts like those typically in Bionicle, of softer plastic (thought to be PETE) than a 2 x 4 brick (made of ABS), the thin clear top layer of harder ABS plastic Pearl Light Gray is difficult to discern. So the most distinct difference between Flat Silver and Pearl Light Gray may be in the lightness or darkness of color. If the part is darker and the intricate edge details of the molded part seem sharper, it's Flat Silver. If it's lighter and the edges seem rounder, it's Pearl Light Gray. (All of which is evidence that, for such softer parts, Pearl Light Gray and Flat Silver are just production batch differences and the particles in the plastic have settled differently.)

Metallic Silver not only has no such thin clear top layer, it seems grainy above the surface and is more reflective than Flat Silver.

Rust is not merely a slightly darker red than Red. There is a distinct orange component to it. In it's earliest appearances as a Boat Hook, Flag, and Bar, it is believed to be a manufacturing fluke. But while the flag and bar parts are evidently made of a plastic that differs from the typical shiny rigid ABS, the Boat Hook components are much less obviously so, and may well be normal ABS. Years later, when LEGO introduced the Throwbots/Slizers, two parts, a gear and worm screw, neither of typical ABS, reproduced that much earlier orange/red color and did so while in sets that also used plainly Red parts. Still later, when those two parts appeared in other sets, they retained that orange/red color. LEGO has produced other parts in Red that are not the typical ABS yet they seem singularly unconcerned about color matching these two parts to Red. If Rust is a fluke, it's a longstanding one worthy of consideration as a color on its own. And, LEGO has also produced a Duplo cow that, while it is not the same color as the System and Technic Rust parts, is decidedly far enough from Red to warrant a different color label and can by itself be considered worthy of the label "rust". In fact, Duplo parts have a history of being 'close to but not quite' normal colors. Which brings us to...

The earliest appearance of a color identifiable as Lime was in Duplo and Fabuland parts and it is slightly darker than the more common Lime found years later and starting in Star Wars sets. Similarly, Light Lime is used to refer to a color typically found in Scala sets but Duplo has its own version which the catalog admins fondly refer to as lemon-lime. Neither the darker earliest lime nor the distinct lemon-lime have been deemed worthy of a color label of their own however. This is due partly to the fact that, so far as we know, items in these colors are available only in these variations of the colors, and are not produced, for example, in both lemon-lime and 'normal' Light Lime. And it is due partly to the relative rarity of the parts involved, the associated resale market size, and the confusion that is ever present amid a plethora of color names for colors that most people have no firsthand experience with.

Adult LEGO fans everywhere call green by Green. But LEGO's catalogs, when offering bulk brick sets, often call this color Dark Green. And reportedly, our Bright Green they call Green. Green, some color green, was one of the earliest colors produced by LEGO though it wasn't used in brick type elements until years later and it did change from its earliest appearance. Maybe they are still thinking of that earliest green as Green.

On the other hand, Adult LEGO fans everywhere are still thinking of LEGO's earliest brown and grays as Brown and Grays. In 2004, LEGO made a deliberate, significant, announced change to the appearance of these colors. And while fans refer to the new colors as Reddish Brown, and Bluish Grays, LEGO has made no distinction in the naming of their bulk brick sets and continues to label them with Brown and Grays.

Something else LEGO did around the same time was standardize their spelling of "gray" or "grey" to Grey. But BrickLink had already and long since standardized its own usage to Gray.

The label Medium Violet covers two colors, or rather, two versions of one color. The older one is bluer, the newer one is redder. Don't confuse one for Violet or Light Violet or whatever, in comparison, if you've got both. The older one tends to appear in Duplo flowers, the newer one tends to appear in Duplo promotional bricks, and, System bricks available at LEGO's various direct retail shops. So far, the parts made in each version of the color have only been in one version of the color. And we'd like to keep the Color Guide list of colors to a minimum. The change by LEGO was deliberate and occured about the same time as the change in grays and brown. There came many new pastel colors, some being only in very limited release (an orange) or use (Medium Violet), but most being new versions of existing lighter colors which were widespread enough to warrant new color names on the list (Bright Pink, Bright Light Yellow, Bright Light Blue).

Although Metal Blue falls under the Pearl Colors, it was named "Metal Blue" rather than "Pearl Blue" so as not to be confused with the slightly pearly blue color (not officially recognized by the BrickLink catalog as a distinct color) found on some Bionicle Rahkshi parts. Because Metal Blue does exhibit a hint of swirliness, it is called "Metal Blue" rather than "Metallic Blue" to distinguish it from the Metallic colors, which lack any swirliness.