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By their very nature, typical achromatic refractors would be no good with most types of imaging, including RGB. As noted already, the blue end of the spectrum is the most problematic. Achromats fail to keep blue in a tight line, the blue spectrum is spread out, with violet end the least tight, or more scattered. No matter of blue filters or violet minus filters will help here as the entire blue spectrum is spread out from the start.
Then, depending on the individual optical arrangement, eg, 80mm, 120mm, etc, focal length/focal ratio, the glass used, the number of elements, other colours of the spectrum will respond differently too, some blurry others pin sharp. This too will determine the ultimate quality of APO's too, and this has been noted here too.
The achromatic refractors that work best for imaging are oil spaced long focal ratio instruments, and can be doublet or triplet. These are able to control spectrum scatter very well, as well as high quality APO's. But these instruments today are rare as they are typically larger aperture too, making them both monstrous in size and price. Smaller, high quality APO's have made these redundant.
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