What model of pacemaker do you have? I'll talk to my friend in Cardiology (I crack myself up, but I will speak with her).
I would also call the pacemaker manufacturer and see if there are any reported issues. I would imagine these considerations are accounted for by the FDA, FCC, etc but obviously this isn't something you want to make assumptions about.
Light colored leather usually isn't a huge issue but with time (years and years) black looks better. The real problem is the carpet. Light colored carpet is going to get dirty unless you live in the desert. If you live in an area with snow (and therefore salt and sand) it'll be disgusting after the first storm. Rubber mats help, but there's still the carpet that runs along the center console to get dirty.
I'm not sure what the new ES interior looks like specifically, but I imagine being a Lexus/Toyota product if the leather is "parchment" then so is the carpet. If they do black carpet with the tan seats I think that would be a much better option. Plus the contrast makes the interior more interesting. I don't think they offer that though.
I'm a fan of brown leather, I think it looks great. Otherwise I'd say go with black unless you live somewhere with no dirt, rain, or snow.