-Use the color picker to select the color you want. (This does not work for black or white.)
-Create a new layer.
-Make a square selection on the blank layer.
-Fill the selection with your color using the paint bucket.
-Under the "filter" menu at the top of the screen, select "Pixelate" and "Color Halftone".
-I use the default settings for max radius and color channels, but you could change the radius is you want.
Photoshop will update your color the the new colors in a series of circles. You will have a bunch of colors to choose from now that all go together.
The colors will probably be too bright. You can adjust the hue and saturation by going to "Image" in the menu at the top of the screen, then select "adjustments" and "hue/saturation". Drag the sliders for "hue" and "saturation" until you get a color scheme that works for you.
When you are done you can deselect the square and save this layer in your document. Whenever you need to match a color or get a new color, you use your color picker on this layer. You will always get an exact match and everything will be coordinated!