Pixelate - Colour Halftone not working

You can use the Rasterize effect to convert to greyscale before the Halftone effect.

Color the result after expanding, or if the Rasterize effect was very high res, use Image Trace to convert to vector.

To create the color afterwards what you can do is just draw a rectangle and apply the black and white halftone raster as an opacity mask

Window > Transparency

Your best option is probably to create the effect in black and expand it then add the color afterwards if that makes sense.