Experiment. Along one stretch of joint, apply a new layer of your preferred caulk directly over the existing caulk. If you like the result and the new layer holds up, complete the other walls. You are most likely to succeed with this approach if you first give the area a really good scrubbing to remove any soap scum.
If the new caulk does not stick, it could be that the old caulk was silicone caulk: nothing truly sticks to silicone (including silicone).
Regardless, if the result is not to your satisfaction, then scrape/peal/clean the joint really well before re-applying new caulk.
FWIW, the guy that did my old bath also used the wrong color caulk. As I get older, I'm learning not to trust contractors to handle any aesthetic decisions. :)
(If, on the other hand, you are asking about grout, then that's a whole other question and likely will involve a dremmel tool and a whole lot of dust.)
UPDATE:
Oh, one more tip while we're at it. If you are wanting to match the caulk to the grout, note that many grout suppliers also sell sanded caulk color-matched to the grout.
There are two primary uses for caulk:
- aesthetic gap filler
- functional moisture/air barrier
For the latter, you'd see that wherever you need to waterproof joints (shower panels, exterior windows, etc.)
For the former, it can really be used in any situation where you have two pieces of material meet. Typically this is done when two different materials meet and you want to cover a small gap. Example would be where door trim meets the sheetrock. Higher end finishing would have that caulked before painting.
So, as for these shelves, it's really up to you. One option is to caulk it. Another would be to attach some trim moulding.
As for how to properly caulk, the basic method is:
- cut tip to size
- apply to joint
- smooth to finish
There are tools to smooth it, but I prefer to just use a wet finger (and plenty of paper towels to wipe finger off as you go...)
Note if the gap is too wide, you may need to fill the gap first. I've used foam weather stripping in the past for this.
Best Answer
The entire point of the fire blocking foam is prevent/slow fire from spreading from one area to another; it has less to do with the electrical/plumbing between two areas than the hole that was created to allow them through. As long as you have fire grade blocking to the prescribed thickness, there is no issue with other foam on top of it.