Updated November 26, 2010. This post is a supplement to my Color Chart.
My first custom color mix is shown below on the left. I call it B2000. It's my name for a very pale blue in the B2_ color range. For comparison, the chart includes some similar pale colors (ending in 0's) B000, BG000, and BG10, and also B21, B24, and B26, a few of the darker colors from the B2n grouping. My B2000 recipe is 1 part B24 and 24 parts colorless blender. I could have made it with B21 instead (using more ink and less colorless blender).
Added April 17, 2010: Posted below on the right is my custom color YYR2000, made with 1 part Y11, 1 part YR20, and 3 parts colorless blender.
Added November 26, 2010: I made custom color YG6000 (not shown) with 1 part YG63 and 5 parts colorless blender.
Added September 18, 2011: Sometimes R29 is too dark and saturated, but the lighter R2n's weren't what I wanted. For example, I love R24 but, to me, it looks more orangey than R29. So I made custom color R290 (not shown) with 1 part R29 and 1 part colorless blender.
Caution: colors can vary with your paper, and the computer may not look true to life.
In some cases, the Copic users who mix custom colors were creating paler colors in the 0 series (paler versions of the existing B000, Y000, and so on). As of July 2009, there were new extra-pale colors that might be perfect for them. But what I wanted with B2000 was something in the B2n grouping, which is grayer and less bright than B000.
Marianne Walker's I Like Markers post here explains how to mix custom colors. To understand the Copic color numbering system, look at her Copic Glossary, near the bottom right of her blog pages, for descriptions of Color Family, Color Saturation, and Color Group. Her post here tells more about the color numbering system.