Ink layer order

There is no surefire order for layering riso inks when overprinting—unlike something like screenprinting or offset which have hard and fast rules (usually light to dark). However, there are multiple rules of thumb which can provide some guidance.

General logic

The standard modes of color order are:

1. Least coverage to highest coverage — This is most important in the center strip (which is prone to roller marks) and the lead edge of the page (which is under the highest pressure when feeding). By laying down low coverage layers first, the feed tires will pick up less ink for subsequent passes. If there is no ink at all in the center strip (perhaps it is the blank inside margin of a 2-up or 4-up layout) the next color can be put down immediately (assuming there are no wet-on-wet considerations—see below).
2. Opaque colors before transparent colors — Some riso inks are more opaque than others, such as Cornflower Sunflower Mint Bisque Metallic Gold etc. These are often printed first, so that they don't block out the colors beneath them (unless that is the desired effect). See ink opacity for more information.
3. Pinks and reds last — The warm pigments, especially Fluorescent Pink and Bright Red tend to be the slowest to dry. So doing these towards the end of the order will decrease the likelihood of messy roller marks.
4. Yellow last — Due to something in its chemical composition, Yellow tends to act as a resist to other ink colors printed on top of it. The colors get slightly muddier, losing their vibrancy. This property doesn't seem to be observed in any tones of yellow, or any other inks.
5. Desired ink for "color grading" on top — In overprinted colors, whichever color is put down last will often shift the hue slightly in that direction, creating a form of "color grading" (i.e. in a CMY separation, if the yellow is printed last the image will be slightly warmer, if the cyan is printed last it will be cooler). This can be tested by changing the order of a multicolor print, and observing the difference in the final outcome.

Depending on the unique challenges of the print, different guidelines may weigh more heavily than others.

Additional considerations for two-drum machines

Two-drum machines add two additional complications:

6. Principle color first — A two-drum machine prints wet-on-wet, the ink from the second drum is printing directly on top of the wet ink from the first drum, before it has time to dry. As a result, this ink will transfer back onto and into the second drum, slowly muddying the color in the overprinted areas—while the first print may show each color cleanly, as the printing progresses, the overprinted areas will gradually shift more into the tint of the color in drum position one.
7. Critical registration colors paired together — The registration between two colors in a single pass is very consistent on a two-drum machine—however the position of the two colors on the page still varies from sheet to sheet. This will become visible if a third or fourth layer are added. So it's important to pair together the colors who need to be closely registered to each other.