I have no foolproof formula for picking out fabrics. I just encourage you to know what you like and seek that out. I suspect that part of the reason people have trouble choosing fabric is a lack of confidence. Most of us make decisions about colors and prints all the time. We decide what clothes to wear and how to decorate our homes. If you're feeling overwhelmed by your fabric options, stop to consider what you like. What colors have you painted your home? Do you wear mostly solid colors or prints? Know what you like and, if you're not finding it, look somewhere else. Nobody wants to spend hours working on a quilt with fabrics they don't really like!
The Mod Sampler is based on 6 pairs of fabrics (12 total fabrics). It will be most successful when 6 of the 12 fabrics (half of each pair) share a particular characteristic, while the other 6 (the other half of each pair) share another. Ideally, there will be some contrast between the 2 halves. Some examples are: light/dark, color/neutral, warm/cool, print/solid, and multicolored print/monochrome print.
I've put together several groups of fabric that I think would look good for this project and I'm just going to talk through different ways that I think each could be used.
This first example is for a monochromatic color scheme using light and dark pinks. Take a look at the prints I've chosen. They're all small to mid-sized prints that are color-on-color or color-on-white/ecru. Notice how there are no sharp angles in any of the prints. Notice also how the solids that I've paired with each print vary. Some of these pinks are more orange and some are more violet. This will make for a more interesting composition.
For the Mod Sampler, I could use the 12 prints (6 light and 6 dark), the 12 solids (6 light and 6 dark) or any 6 solids with any 6 prints. I would use a white or snow sashing for this one and, whichever prints or solids I ended up using, I would use 2 of the lighter choices for the back pieces and one of the darker choices for the binding.
Here's an analogous color scheme using greens and blue with a little bit of yellow. Instead of dividing these by color (which would be difficult since they're all multicolored prints) I've divided them by type of print. One group is geometric/tile/scrolly prints and the other is floral/botanical prints. I've also picked out 2 sets of solids, one is all blues and one is greens and a couple of yellows.
The options for this set of fabrics are pretty much the same as for the first group: Use the 12 prints, use 6 prints with 6 solids or use the 12 solids together. I would choose sashing for this one based on how bright I wanted the finished product to be. If I were looking for a very sharp, bright look, I would go with white. For a more subdued look, I would choose natural linen or a neutral colored cotton like Kona Stone. Since most of the prints are of similar value, I would probably choose one geometric and one floral for the two back pieces.
Rainbow color schemes are always fun for me. This one uses 6 colors and pairs a light and dark value of each. I've shown an example above of how this can be done with prints or solids. Of course, you could also pair 6 prints with 6 solids, as I've described in some of the examples above. If I were pairing solids with prints, I think I might go for more contrast by pairing the dark solids with the light prints and vise versa.
A rainbow color scheme would also be a great way to make the scrappy version of this quilt. You'd just need to gather scraps in 6 different color groups and make one set of blocks using each color. If you wanted to keep some uniformity among your scrappy blocks, you could use the same solid for each set of six but use a variety of print fabric scraps on each.
This is another one where I would choose either white or neutral sashing depending upon how bright I wanted things. For the backing, I would pick two different colors but keep them the same temperature (e.g. pink and orange or blue and violet). For the binding, I would probably go for "bright but light" like the light yellow or light aqua.
I think the very simplest thing you can do for this project is to focus on 2 colors that you love. In this case, I've paired red prints with aqua prints. Most of the prints are small to medium sized and most of the prints are just red and white or aqua and white. There are two exceptions (the Flea Market Fancy print with the espresso eyelets and the Lotus Wallflower, which is quite a bit larger than the other prints) but that just helps to keep the composition interesting. What you want to avoid in a composition like this is adding fabrics that have too many other colors. Make sure your fabrics read "aqua" and not "aqua with yellow flowers and little ducks" (or something like that).
I would use white sashing for this one, two of the aqua prints for the backing and a red print for the binding.
I have been hoarding black and white prints for a while now. I love the way they look with bright colors and I think they could be used with this pattern paired with either bright colored solids or monochromatic bright prints. I would definitely use white sashing for this one. The back, I think, is a little trickier. I would probably end up choosing two of the black and white prints with lots of white background and then using a bright color like chartreuse or hot pink for the binding.
Another easy strategy for this pattern is to buy fabric from a single collection or designer. This layout uses prints from Anna Maria Horner's Garden Party and Good Folks. I've again separated them by type of print rather than color and I've selected both a deep and a light set of solids.
This is a set of fabrics where I would definitely not use white or off-white sashing. If you look at these prints you'll notice that there's almost no white in them. If I wanted something bright with a lot of contrast, I would choose a solid color like gold, tangerine or teal. I think a natural/neutral sashing would also look great.
Because I cut most of it up into little patchwork pieces, I don't usuallybuy the larger scale prints from collections like this. The back of the quilt, however, would be a good place to showcase large-scale coordinates that you haven't used in your patchwork blocks. (And there's no problem with using those large-scale prints in your blocks! It's a matter of personal preference.)
Of course, you don't have to use quilting fabric at all! I'm a big fan of vintage linens and think they would look great in this quilt either alone or paired with some light, bright solids and white sashing.