Choosing the right base coat color is very important in hydro dipping. There are the standard choices of base coat colors for films and then there are the custom choices as well. The standard base coat colors for hydro dipping are white and sometimes tan for a camo design. But your base coat can drastically change the final look of a dip.
Most films are designed to be applied over a white base coat and that is one reason why you rarely see white printed on a film. The white base coat is what makes the printed design show in its true color by design. It makes the blue ink stay blue and red ink stay red after you dip. This is because the inks usually have some degree of opacity to them that allows the base coat color to impact their true color when dipped. So if you are wanting the design to look as intended and usually shown on the product page of where you are purchasing the film, then using a white base coat is usually the best choice unless it is specified that another color was used in the photo.
For camo designs this can change. Maybe you like the camo design but you don't want it to be so bright. This is where using something like a tan or light khaki as a base cot comes to play. Using a base color like tan or a light khaki can remove some of the brightness of the design, but still keep the design true to the original look. You can also play around and test with other colors like yellows and such to adjust the coloring a little more.
You can get truly unique looking designs by going all out and using other colors for a base coat.Take a look at this rifle below. It was dipped with our desert digi camo (left side of image) over a dark grey base coat.
AR-15 with a dark grey base dipping in Tan Digi Camo.
Even though the film was printed with the tans, none is shown in the final outcome. That is because the ink in the film and the base coat color combined to make a new color. It goes back to grade school when you learned about the color wheel. For example, if you dipped blue flames over a red base coat, then you are not going to get blue flames on the finished product. You will more likely get flames in the shade of purple with maybe a slight blue tint here and there.
Now when it comes to designs that are only black and transparent, you can get very creative with these. Black ink usually is not impacted by the base coat color unless the film gets stretched when dipping. This allows you to use basically any color for your base coat or even multiple base coat colors.
Ghost Flames design over a yellow, orange and red faded base.
Skull Design over Metallic Synergy Green and Metallic Blue base.
Bottom line...Choose the right base coat color for the job and what the finished product needs to look like, but don't be afraid to test and try something new. Before you go to use other colors as a base coat, do a test dip on something else like a speed shape or blank light switch cover. This will not only let you and/or the customer know what the finished product coloring will be, but also let you know for future dips what certain color base coats do to certain colors of ink in a design.