Each individual element of a pixel string is a group of LEDs (usually red, green, and blue) along with a chip (an integrated circuit, or IC) to control them.  Not all pixel strings and chips present the colors in the same order.  While industry standard is RGB (red, then green, then blue), a pixel string could present colors in any order.  Particularly common are GRB (green, red, blue).  The RC4 DMXpix and LumenDimPix can re-order the colors so that any string can behave as RGB, regardless of what the actual hardware does.