Pen color defines plotted line thickness:

In earlier times when plotters used real pens AutoCAD colors were used to determine which plotter pen to use to draw each object.  Now, with ink-jet plotters, there are no separate pens, but the concept of virtual pens is still retained by AutoCAD.  The AutoCAD Plot function allows any line thickness and any color to be related to any AutoCAD layer definition.  This provides great flexibility but can also lead to confusion.  MYCADKEYS recommends a direct relationship between pen number and color number (follows ISO standard) i.e. color 1 plots pen 1, 0.18mm, color 2 pen 2, 0,25mm and so on. It may be necessary to make adjustments to pen thickness to allow for plotter and paper characteristics e.g. 0.25 may become 0.2 etc. but should not vary by more than 0.07mm and the order should always remain constant. 

Much of what we plot is black on white paper requiring lines of differing thickness to differentiate the significance of linework, so the first few colors are assigned to virtual pens with black ink and the required line thicknesses.  The remaining AutoCAD colors, (of the total of 256) can then be assigned to real colors.  All of this is defined in Plot Styles.

The MYCADKEYS layer naming system encodes pen colors in each layer name, but rather than using the three characters, required to list numbers 001 to 256, it uses only one character.  This may be a number from 1 to 9 or a letter from A to Z.  When using extended layer names. EXTNAMES = 1, it is also possible to use several other characters such as $, @.  Bearing in mind that in normal usage nothing like this number of pens is used and the single character is adequate for most purposes.

In addition to the standard color/pen-thickness range to plot in black the MYCADLAYERS' default setting provides for two other optional color/pen thickness ranges; one for darker screen versions of the first (also to plot in black) and another full screen color set to plot a selected range of true colors selected for clarity and range.  The last few pens (color numbers) are used to define a range of grey shades.   The MYCADKEYS file that controls these colors is easily edited to create any preferred arrangement.

The list of pen-code to color/thickness assignments shown in MYCADLAYERS' layer naming tables conforms generally to the ISO standard but can be customized to suit preferred office practice.

As a general principle, but not an essential part of MYCADLAYERSI, the author recommends a black screen for visual clarity.  White lines are reserved for unresolved data, clouds, attention getting temporary notes etc. grays are used for reference material such as grids, text, dimensions etc. and all confirmed buildables are drawn in red, yellow, green, cyan, and blue.   Color six, magenta is reserved for work in process and later changed to one of the first five colors.   Color 8 is used also for hatch and reference information such as text and dimensions and non-printing information such as view-ports. This ensures that the model is visually dominant over the supplementary reference information.  Color 9 may be used for extra screen emphasis or more important reference information.  Colors 251 to 254 (grays) are useful for shades since their appearance in AutoCAD’s Layer dialogue box approximates various shade densities,

Table of Pen Nos. Color Nos. and line Thickness:

MYCADLAYERS comes configured with color, thickness relationships similar to the following.  This table is fully customizable by the user or may be partially edited to suit particular jobs.