I've got a web form with top-aligned labels. Will bold text make it easier for user to fill in the form or will it be it more difficult for them?
Do you know any studies about that?
LukeW has a short and concise study on the different alignments of form labels.
In my opinion, for top aligned labels, you can either:
Use a font that has multiple thicknesses [thin, light, regular, bold, title]. This will give you the freedom to chose your font thickness without making it overly bold and unreadable.
Using Gesalt's principle of grouping, if you keep your text-textbox close to each other and the text-textbox pair a little further from the next text-textbox pair, your users should not have any difficulties because form filing is a practice too ancient to surprise!
According to sitepoint - top-aligned labels "are harder to scan, as they are separated by fields". Relying on this information one can argue that font weight/color does make difference, but minor.