All channels from explosia are exported by default (since C4DtoA 3.3.2), so you should be able to just read the color channel (e.g. in a standard_volume or volume_sample_rgb shader).
If it does not work, please send us a scene and I'll take a look.
I cannot get that to work..
Please take a look
Wetransfer link to project file...
Indeed, the Explosia FX integration is missing support for the color channel. I'm going to fix this in the next release. Meanwhile, if you need the fix right now, please request a custom build via support@arnoldrenderer.com.
Fixed in C4DtoA 3.3.7.1. You can now sample the color channel via the volume_sample_rgb shader.