I recently bought Capture One from a sale, and as my first exercise I re-created a split tone I had done in Lightroom. I found two ways to do a split tone in C1; below the fold guide for both with pros and cons.
The first and most obvious way is to create a split tone using the Split Tone dialog from Black & White tool under Color tool tab,
Using the same settings for highlights and shadows as I had in Lightroom resulted in a split tone which was too strong. I easily fixed it by taking down the saturation. Below comparison of the desaturated image before and after applying the split tone, along with the settings used.
While the Split Tone dialog is easy to use, the main shortcoming is that you can't adjust the mid-point of the highlight and shadow. When you have a low-key photograph like in the sample this can result in a tone which is off, and you have limited control.
For greater freedom, I got a tip on reddit to use the Color Balance tool instead, which is right above Black & White tool. Color Balance is an intuitive tool to adjust saturation, lightness, and hue of shadows, midtones and highlights separately.
I could easily create the split tone I wanted with this: for shadow tone I dragged the center circle towards the blue tone I wanted, and adjusted saturation using the arc next to the color circle. Similar approach for highlights. On the right the settings I used to create the same split tone.
But I needed more control in pinpointing the cross over point of shadows and highlights. For that I created a layer for shadows, and used luminance masking to only affect shadow areas with my split tone. I repeated this for highlights.
How to do that is beyond the scope of this blog entry, and I believe works only in version 12 or later. Short version is to first mask the whole image, and select the desired luminance range where you want your shadows to cover using Luma Range dialog, and create another layer for shadows. Here's a great tutorial on Fstoppers on how to work with masks in C1.
Below the Luma Range I used to separate highlights, and the updated highlight tone in the Color Balance dialog.
To recap, there are two ways to do a split tone in Capture One:
Split Tone tool under Color Tool tab
Pros: quick and easy to use
Cons: can't control the midpoint of the split tone
Color Balance tool also under Color Tool tab
Pros: very powerful and intuitive to use
Cons: A minor shortcoming is that there's no way to enter your colors numerically, so you have to eyeball your results. If you need to use the same split tone elsewhere, you can create a preset, though.
Although you can create a basic split tone with the Split Tone Tool, you have more control and freedom using Color Balance tool combined with layers and luminance masks.