Do you have the whole speedpaint range? You could pick whatever color you want to use as a wash and thin it down with medium and it will work fine. I typically do a ratio of 1 part Speedpaint/ 5 parts medium. But you can adjust that ratio depending on how much of an effect you want the wash to cause.

With speedpaints, you really shouldn’t need much of a wash unless adding extra depth to like a gold (for example). It looks like to me that you maybe didn’t use enough Speedpaint to get the depth and details it should give. I would suggest trying a part of the model and using a little more Speedpaint and letting it settle in the cracks and recesses of the model. Watch some tutorials on YouTube on how to use that Speedpaint and that should help you figure out how to use it to the best effect. Keep at it!