The colour or image of a material starts with the diffuse. This can be a solid colour or with a map. A map is used to add a picture to the material. Select assign to open the media folders, in here you can add images to use for all the different functions. For a map, you can use a .jpg in 1024×1024 resolution.
To have more detail than a single 1k map you can use tileable textures and scale the UV map accordingly.
The Diffuse colour can also be used to add a colour overlay onto the map.
The opacity of a material can be changed in the transparency tab. To change the overall transparency, of the material, change the value of opacity to a lower one (1 is fully opaque and 0 is completely transparent)
You can also use a map to selectively change the opacity based on the texture. In an opacity map, black is transparent and white is opaque. All the grey values in between are values of transparency.
Important is to set the toggle next to transparent to on.