If you have a really good reason to do so (otherwise, just for fun) you can build the ability in a project.
Here's an example:
You can drag the RGB 0-255 group to your Favorites folder in the Motion Library for reuse.
All it is, basically, is 3 number generators (animation turned off) linked to the red, green, and blue color values and scales these to 2.55 times their value. The way Numbers works is the value will be 100 times what you might expect (you can choose different display types -- my preference was Hexadecimal -- aka HTML color).
It's not that difficult to retrain yourself to see number values as decimals or percentages. The range 0 to 255 comes from the Hexadecimal representations translated as base10 values. It really doesn't matter what the "counting" system is, the 3 values represent magnitudes and RGB color is relative to the combination of magnitudes (including saturation and brightness). If any one of the 3 values is 0, the saturation is 100%. All 3 values equal to each other, the saturation is 0%. All colors are the combination of any 2 values with the third value "contributing" to the desaturation (or 1 value and the other 2 equal to each other contributing to desaturation). The maximum of any of the 3 values determines the brightness. If you know your color wheel, that pretty much covers it.
Ok... cool. You don't need the Motion project.
What you do is for whatever object you need to color match, left click on the color swatch and you'll get the Apple System color picker. Click on the second icon on the top (Color sliders) and select RGB Sliders from the dropdown if it's not already set to that. Use the Magnifying glass to pick the color from the illustration. The Values are shown in 0-255 format.
It may be an issue if your Illustrator artwork was saved as CMYK... If you can, go back and export it with RGB color model.
In the past (I cannot verify right away with Motion 5) Motion has had a minor issue with obtaining exact colors from a "pick" - if you know the exact color values, then you can type them into the appropriate spaces on the color picker and the object should be correctly colored. (I would venture a guess that values would be averaged from adjacent pixels if you were not exactly over a specific single pixel... but that's just a guess.)