Colour change refers to the decision by LEGO to phase out three common colour elements (grey, dark grey and brown) to be replaced by new versions of these colours. The change was initially begun in 2004. Almost all models released since early 2004 featured the new colours, although some elements (particularly minifig accessories) were still released in the old colours.
The two new shades of grey had a bluish tint to them when compared to the "warmer" pre-2004 greys. The new shade of brown had a red tint when compared to the pre-2004 brown. The new grey colours are commonly referred to as bleys or blays. (Bluish + Grey = Bley)
This act caused widespread dismay and complaint amongst many AFOLs, some of whom refuse to purchase models containing any of the new colours. Many AFOLs were upset at not just the removal of three staple colours from LEGO's palette, but the fact that those three colours were replaced by ones that were just similar enough to be noticeable. A shade of mauve, often called "sand red", was also deleted in 2004, but received no replacement colour, and therefore its change did not attract nearly as much attention. Discussion of the change sufficiently dominated some LUGNET newsgroups that a new one, lugnet.color, was created to provide an outlet for the anger at the grey change.
One of the continuing problems that AFOLs have with the new colours is that they are inconsistent with each other. A present day set may contain multiple 1x4 blay bricks that are not all identical in colour. While parts were known to have some colour inconsistency in pre-2004 sets, there are much more distinct differences in post-2004 sets.
Unfortunately TLC's colour inconsistencies are not limited to the colours introduced in 2004. A possible reason for this could be their change in plastic production methods. For decades, TLC had made the premolded plastic pellets precoloured. Soon after the colour change, they began adding the "master batch" colour tinting to the hot clear plastic as it was being injected into the molding machines. (Adding tinting at the point of molding gave TLC the option to produce much shorter production runs of a piece in a particular colour.) Soon after this method began, many pieces were found to be slightly translucent due to not receiving enough colour tint. This problem with translucent pieces has since been remedied.
Here you can see the difference between the colours that changed in 2004.
|Pre 2004 Colours||2004 & Later Colours|
|Light Gray||Light Bluish Gray|
|Red: 197||Red: 200|
|Green: 192||Green: 203|
|Blue: 184||Blue: 204|
|Hex: C5C0B8||Hex: C8CBCC|
|Dark Gray||Dark Bluish Gray|
|Red: 127||Red: 118|
|Green: 120||Green: 121|
|Blue: 112||Blue: 126|
|Hex: 7F7870||Hex: 76797E|
|Red: 109||Red: 116|
|Green: 60||Green: 66|
|Blue: 15||Blue: 44|
|Hex: 6D3C0F||Hex: 74422C|
colour table for info on colours and links to a lot of other colour resources.