What is the best way to seal small grout cracks in the shower?
Answers:
Aesthetically, you are better off taking a hand grout saw and cleaning out the affected joints and regrouting. I've never seen any sealant that looked good pasted over the top of grout joints, clear or opaque.
If you are talking about sealing corners, that is another issue.
The only product that I will use in a shower is urethane caulk. I like the Pro Series Quad in white, available at Lowe's for about $4.25 tube. Silicone is often recommended, but in my experience it is a defective product for tile joints. Make sure that your joints are clean, dry, free of mold , soap scum, and any loose grout. Use mineral spirits to get a clean surface that will allow the caulk to bond. Run a nice bead with your caulking gun and work it in with your finger on each joint as you go. This is an oil-based product and does require mineral spirits for clean-up. If you do a good job and get a nice seal, this is a life-time application that will not grow mold.
Other answers:
colored caulk
colored caulk