Can you use a custom colour in a table in Framemaker?

for example, R 123 G 85 B112

I might add that:

You'll use the

Model: RGB

submenu for that.

And this menu is percent based, so you'll need to convert your 0-255 values to 0-100%.

And these are uncalibrated colors (you can't specify a color model, white point, gamma, or anything else). But you can enter, for example sRGB values, and "tag images for color management" later in PDF rendering (with the sRGB color model elected).

Use

Print As: Process

Overprint: Overprint

unless you have a known reason to use other settings.