It makes life easier still if the palette is nearby to the painting surface and if it's more or less parallel to it. The near position reduces the amount of wasted effort between mixing a stroke and applying it, and the parallelism ensures that a mixture of color on the palette matches a corresponding stroke on the painting.
I prefer to have the palette on a hinged surface directly below the painting. I often tilt the palette up a few degrees so that it's perpendicular to my line of sight, and also to reduce the tendency of some paint to run or drip. The Open Box M pochade easel that I'm using allows for that configuration.
I use white freezer paper (polyethylene coating) for my mixing surface, which makes for easy cleanup, and the white color allows for more accurate judgment in transparent mixtures..
Over the years, portrait painter David Kassan has been developing what he calls a "Parallel Palette" which sets up next to his easel on its own tripod. He has been refining the design and, together with a partner, has just launched a version on Kickstarter.
Here's Max Ginsburg using a prototype. The palette attaches to its own separate support, not necessarily to the easel directly. For most people this would require carrying a second tripod on location.
I have not seen or tried out the product, but here's my take on it based on the Kickstarter pitch. The gray plastic box has a rather small (6 x 7.25 inches or 15.25 x 18.5 cm) mixing zone in the center. Following the rule of thumb that the mixing surface should be no smaller than 25% of the area of the painting, I would not recommend this palette for any painting larger than 12 x 16 inches.
The mixing surface is made from clear plastic. It can be removed for cleaning—as long as you clean it before the paint dries. Because of oil paint's powerful adhesion to plastic I would suspect that it's a lot harder to clean than glass would be because the plastic can't be scraped with a razor blade.
On the upper area and sides are gray areas for the paint from the tube. There are shallow ledges that are supposed to reduce the dripping paint. But some paint is very oily and will inevitably drip on any steep slope. Even with those ridges, I would be concerned that some paint or oil will get into those slots that are needed to hold on the mixing surface. So it would be wise first to squeeze out any oily paint onto a separate absorbent surface to extract the excess oil before loading the palette.
If piles of paint are allowed to dry in the spaces between those ledges, I don't know how you'd clean that out; on a glass palette, you could just scrape it off with a razor blade scraper.
In the bottom section are four elastic bands designed to strap in cups or jars for medium and solvent; I don't know if those cups are included. If you don't use that space for medium, you can use the narrow shelf to hold an extra paint brush or two. Keep in mind that a normal oil brush will stick out two inches on either side of the palette. To hold more brushes, most painters use a more substantial brush tray or folding brush holder. What I do is drill graduated holes in a horizontal panel to manage unused brushes.
A clear plastic lid fits over the whole thing, so that you can safely carry the wet paint around or pop it in the freezer overnight.
The eventual retail price for the Parallel Palette will be $139.00. By comparison, a John Pike watercolor palette (with cover) sells on Amazon for less than $30, and could easily be adapted with a scrap of wood and a T-nut bracket for the same use.
Again, I haven't tried one out — so this isn't a review, just some comments based on what I've seen in the marketing materials.