Make three circles.
Fill one with pure red (a value of 255, 0, 0), fill one with pure green (0, 255, 0), and fill one with pure blue (0, 0, 255).
Create a new adjustment layer above your circles.
Apply Hue/Saturation to the adjustment layer.
Crank the saturation down to the lowest level.
All three circles are now grey. The exact same grey. Move your mouse over them and note the color value (127, 127, 127). If you selected that grey value to color it, which color would you make it? You won't get it correct because they are all three very different colors.
I hope this shows you that your idea is a flawed one. You could use color keying to select all of the same exact shade of grey, but you won't be able to colorize it that way.
You could use the rotobrush to select certain areas, but you'll have to try it yourself to see if it helps you do what you want. Please go through these resources on using it first or you'll be frustrated.
If you have a green screen keying is a much better option. RotoBrush is only designed to remove objects from non contiguous backgrounds and it requires training to use. I've been using AE for 20 years and the first time I tried to use RotoBrush I failed completely because the interface is not like any other tool. Go through these resources unless you are doing green screen work: Roto Brush and Refine Matte | CS6
Go through this info if you are using keying: After Effects Help | Keying
I noticed that you are trying to do you matte work on Black and White footage. This requires luma keying. Sometimes you can create a procedural matte using curves, levels, threshold or a combination of the two but you have to realize that you are trying to isolate color values and you have just reduced the accuracy of your selection from about 16 million (the Permutation of 256 things taken 3 at a time) of colors to 256 colors (given an 8 bit original). The selections are not going to be as good and Rotobrush will probably do a better job than procedural mattes because RotoBrush looks for edges.
Both of these resources are available directly from the Search Help field at the top right corner of AE by just typing in the appropriate terms. There's a lot more info available there too.