Turn over the loaf and look at the ingredient list. Bread colour tells you little about which flour makes up the loaf or how much fibre a slice provides. Check the named flours, any declared proportions, and the nutrition panel for the bread you actually buy.
Compare the flours and then the slices
Suppose one fictional loaf lists whole-wheat flour prominently and gives 6 g fibre per 100 g. Its slice weighs 30 g, giving about 1.8 g fibre per slice. Another fictional loaf gives 4 g per 100 g, with a 45 g slice, also giving 1.8 g per slice. The first has more fibre for the same bread weight, while these particular slices provide the same amount. Check the real packet if your slices are different.
Choose for the meal you make
If you buy bread for a work sandwich, read the panel and think about the filling too. A sandwich with a pulse, dairy or other protein-containing filling is a different lunch from plain toast. Check allergens and sodium if those matter for your household. If a listing only shows the front, look at the delivered pack before using an online figure.
The Indian label guide explains how to compare panel bases. The fibre breakfast guide gives other familiar choices when bread is only one part of the week.