Essentially, the reason why wood and your blue jeans are darker when they are wet is because the refractive index of a material depends largely on it's density and whether it's a solid, a liquid or a gas.
Dry wood consists of hollow wood cells that are full of air. Since wood is a solid (consisting mostly of cellulose), there is reflection and refraction of light at the wood/air interfaces within the wood. The larger the difference in the refractive index of the two materials across an interface, the more light is reflected at that interface and the greater the angle the light that crosses the interface is refracted through.
When wood gets wet, then those hollow wood cells become filled with water, and there is less of a difference between the refractive indices of wood and water than there is between wood and air. As a result, less light gets reflected from wet wood (so more light goes into the wood). AND, because of the smaller difference in refractive indices, light travelling through wet wood travels in a straight-ER path than light travelling through dry wood. That means that the light tends to penetrate deeper into the wood and more of it is absorbed by the wood and water, and less of it gets refracted through a total of 180 degrees so that it's redirected out of the wood to an external observer. That observer sees less light coming from the wood, which our eyes see as the colour "darker".
You can prove this to yourself with a piece of paper and a drop of water. Place the drop of water on the paper and watch as the paper absorbs the water and turns darker. This is because as the water surrounds the wood fibers, more and more of the light is penetrating through the wet paper rather than being reflected from it. Now, hold the paper up to a light and you will see that the wet spot is the brightest area of the paper. This is because this similarity in refractive indices between wood and water result in the light taking a straight-ER path through the wet paper, and that in turn means that light behaves in wet paper more like it would in air (or if the paper weren't even there). And, that in turn means that wet paper is more transparent than dry paper.
Now you have a conversation starter for the next wet t-shirt contest you go to. Cotton, by the way, is nearly 100% cellulose whereas wood is typically only about 75 percent cellulose with the remaining 25% consisting of something called "hemi-cellulose" and lignin, which is the "glue" between the wood cells that holds them together.
So, place a drop of water on the bamboo flooring. If it turns dark at the spot where the water was, then there can be no other reason than water penetrating through any factory applied sealer or finish and filling the empty plant cells of the bamboo flooring, resulting in less reflection of light from the wet bamboo.
And if water can penetrate through that factory applied finish, then I would be concerned that other water based liquids, like Saniflush or Easter Egg dye or Kool-Aid could penetrating through that finish and be absorbed into the bamboo. The water in those liquids will evaporate back out through the factory applied finish, but the dye molecules that give the liquid it's colour would remain behind to leave a stain in the bamboo.
(By the same token, you MIGHT be able to leave a drop of bleach on the stained bamboo so that it would penetrate in as well and break up the molecules causing the stain.)