Start at the beginning - your substrate. Make it a moveable one!
The right substrate – something you can move around, place against or stick to the walls, and view it at different times of day and in varied lighting conditions!

Since your paint color will look a bit different on various materials, it’s important for the most accurate color representation to apply the paint layers (yes, including the primer!) on a surface that most closely mimics the actual wall.

Size?
It's important to be able to move these pieces around, so keep that in mind.

Best Practices
Let’s get right into a few practical notes to help you with successful color testing.

1. Avoid Temptation!
   This means, please do NOT do rolled or brushed paint colors right on your walls. It’s a recipe for more confusion and will not give a decent representation of the color. What you need to do is…
2. Create a Surface as large as possible
   Replicate as closely as possible the actual wall (interior or exterior). Ideally: 4’x4’ for exteriors, and 2’x2’ for interiors.
3. Keep it proportionate

Always use materials that are in proportion to the actual application. Walls to Trim, etc.

4. Replicate the planned end result, as closely as possible

Typical paint jobs include primer (for deeper topcoat colors it will be a gray-based primer or one that is tinted to the topcoat color), PLUS two coats of finish color.          

Materials
Many paint companies sell inexpensive "testing colors" in quarts, tinted to any of their colors. But please note (as it does on the can…) that this is NOT to be used as a finish product. Or just purchase a quart of any of the products in actual finish product. The “testing paint” is not intended to be the finish paint, even though it is the same color formula.

The right tool for application
Heavily textured surface: use a thicker/deeper pile roller. If unsure, just consult your paint store for the best one.

Smooth surface: a finer-textured roller is better.
Note: for the smaller-scaled samples, you can use an inexpensive foam roller.

Brushed: Use a brush or a foam roller. If you want to replicate a brushed area, just brush the final coat once you have applied the primer and first color coat.

Replicating spray painted: Use a finer-textured foam roller.
           
Texture
Use material that is closest to your surface texture. You can make one using a board, some joint compound or plaster, or other texturing material. It does not have to be identical but as close as possible is best. Left over remodeling materials are perfect.

Materials
Foam core, lightweight smooth surface plywood, lightweight wall board, even poster board can work. Just prime them all at once. Create some extras for future paint projects, then they will be ready-to-use.

NOTE
Any paper based product must be primed with an oil base primer OR it will warp (think about water on paper). There are good oil-based primers that are easy cleanup and low to no-odor. Look for BIN, Zinsser, etc. or other materials. Plan to properly dispose of any rollers or brushes that you use with these oil based materials.

Small Wall boards
“Small Wall” are prepared paint testing boards available at many Sherwin Williams stores. You can also order larger quanitites of larger boards in a 'contractor pack.'

They come in a two-pack, two pieces of smooth, ready-to-paint (No Primer Needed!) boards that are each 12"x12." They are repaintable, and come with a tape on the back side that is repositionable. 

The advantage
Get a few sets, paint at least two boards of each color you are testing, and stick them up on the walls – adjacent to each other on any angle surface. Wall meeting ceiling; in the wall corners, etc. If you are viewing more than one room at a time, paint up all your test colors and put them in the adjacent rooms so you can see through an opening into the color in the next room.

There also other products that are made for testing paint, that will actually stick to your wall/siding and allow the texture of the surface to “read through” to get an even more accurate color view. I’ve not used these so cannot speak from personal experience, but have heard from clients now and then that they work well.

Visual imaging is another way to virtually test paint colors. While it's best used as a way to see how various colors will look on your building (interior or exterior), it's a great creative process and an excellent part of the "flow" of developing a color design plan.

Obviously since the output is digital (.jpg or .pdf, which of course you can print), the colors are best considered as a reference. However, it's fascinating how accurate it can be when compared to the actual final painted building. I offer this special service,"Designer Color Palettes," to clients of all types. It is much more detailed and comprehensive than you will get in a "diy" online method. You can see some information here: https://www.bjacobscolordesign.com/exterior-interior-color-consultant/designer-color-palettes-visualize/dcp-gallery/

This process is a great initial process for individual homeowners, designers and architects, paint professionals, and HOA groups where reviewing color ideas is important. Then the actual physical colors can be tested to see them "in person."

I hope this information will be useful to you in confirming or continuing to explore color options.