Thank you very much for playing and your feedback.
The blank tiles are currently a bit confusing due to the lack of artwork. The green tile represents a meadow, the brown tile a forest and the yellow tile a farm field (unfortunately in web version, the colors get washed out, which does not help either). These nature tiles are basically fillers, which can be placed in areas, where no track (or hopefully roads, settlements etc. in future releases) can be placed. Any nature tile is compatible with any other nature tile, so it is very easy to fill gaps. But just like in real life, a nature tile cannot end infrastructure, so placing it where a railroad track has an open end does not work.
The total opposite of that is the gray tile: It represents a settlement, which only tiles with itself. In this rough version of the game, this tile can only be placed at the outside of your building area, since it cannot interact with the rest of the tiles (yet; will change in future releases). During testing, we realized that in some (browser and screen) configurations, the brown and gray tile seem to have the identical color. Maybe this leads to your placement issue.