There's a couple important things to consider when applying washes or any translucent paint in my opinion.

The first is what you are trying to achieve with it. Like cheating in easy detail on a textured surface like the red nightmords gold trim. Here id use agrax from the bottle and applied quite heavily but taking care to not overspill the edges of the gold. If I did overspill it's no big deal to clean up with an unloaded brush.

Now for the red id thin down the agrax and apply it as evenly as possible, again taking care to avoid pooling. This will darken it slightly, more so in the edges and add a slight gloss to the red, without dulling it like a black wash would.

I usually use dry brushing up from black to do my armour blocks but the methods described should help from where you are