Go to the brushes panel to find the color / tint options and double click the required preset and goto the colorization and select the required color etc method and click OK
You have four options for the colorization method : none, tints, shade, hue shift. The key color is slightly odd as I notice from time to time that it does not appear to work (in my version) and only seems to start working when you use it in the none option and select the color with the eyedropper. It may work fine for you and select in all methods.
If you select the none method for the color will be as created, the default.
In this, just set that method when you edit the stroke. If you want the stroke to be displayed in pink then go the stroke color in the toolbar and change that to pink and the red will be tinted pink and the black likewise. You can also change the stroke to green and the stroke will then be converted into greens
If you change the method to "tints and shades", the blacks are unchanged. The red will change to a shade of dark pink if pink is used (a tint applied to the gray scale stroke). The brush strokes with this color method works best when the colors are nice bright ones and then you can see some difference between them
Double click the preset in the panel and go to the eyedropper and click on the preview - select the color you want as your key color otherwise it just defaults to a color it decides (normally it seems to be black) and set colorization to hue shift. Click OK and accept any changes. Now when you change the stroke, the key color in the original stroke will be changed to the stroke color. If there are 10 colors in the brush stroke then only that one color changes (not sure why they didn't add multiple key colors or color 1 and color 2 etc). It would be useful if the key color was a drop down with the colors in the path. So if you set the stroke to blue, the key color will change to blue
The most obvious method is to re-color the stroke at source in the AI source file or from the panel. Drag the preset from the panel and ungroup it and then remove the bounding box for the stroke and then change the color of the individual parts of the stroke. Not particularly ideal but you can set up multiple strokes say a brush in red and a brush in green and a brush in blue and then select those brushes as and when you need that particular color.