Hi Mat, I'd also like to share the same suggestion that Zeno made about weathering, and also to offer some tips and advice for you to get started.
Firstly, don't be afraid to apply weathering to your figures. The idea is to make your figure better by making it look more realistic, not necessarily to cover up the good paint job you've spent many hours working on. The amount of weathering you apply is entirely up to you, but if you want to convey realism, you need to weather your figures (unless they're on the parade ground).
To start with, think about the geography, the geology and the season in which your figure is being portrayed. Also consider the status of the person you're trying to convey. Your Celtic Chief is wearing well made good quality warm clothing so perhaps its winter and you could add some snow to the ground effects and add a few fallen red and yellow leaves onto the groundwork to show that its late Autumn/ early winter. If that's the case, perhaps the grass you've chosen for your groundwork could be painted a pale yellow/ straw colour instead of the bright green colour to help sell that seasonal story. The soil could be darker to show that its wet and muddy.
Following on from that, if the season is late autumn/ early winter, then at the very least the footwear is likely to have both wet mud and some dry mud on them. Celts probably weren't big on personal hygiene, so its likely he didn't wash his trousers or his cloak very often either. If so, then his knees and the bottom of his cloak are also likely to be stained and dirty.
You can achieve mud and dirt effects by buying MIG pigments or a range of other weathering products which are designed to accurately replicate real soil and mud from a variety of locations (ie European mud, Desert Sand, Vietnam jungle soil etc). The best and cheapest product I've found are Earth Colour pastels. These are just like chalk sticks, but when you scrape them into a powder and then add some turpentine, they make a fantastic mud wash which dries flat and creates a very convincing dried mud look. If you want a wet mud look, add a drop or two of Gloss Medium to your pastels or apply some diluted oil paint to your mix.
When applying mud you can slap it on or you can splatter and sprinkle it to suit your tastes and the look of the figure. When I want to replicate a splatter pattern, I will sometimes load up a big paintbrush with my pastel chalk mixture and then pull back the bristles with my thumb until the hairs flick forward and spray small drops of pastel mixture onto the figure. With plenty of patience and a steady eye, you can get quite good at applying mud splatters to legs and capes etc.
The shield of you Celtic Chieftain is perhaps a little clean. In my opinion a shield is a functional battle tool used to deflect blows from your enemies weapons and also an offensive tool to push against your enemy in close combat. Either way, the shield is likely to show signs of battle damage and will bear the slashes and scars of enemy weapons. Assuming this Chieftain may have inherited his shield from his father, its possible the shield itself may be older than the man carrying it and therefore likely to be quite distressed and battle scarred.
Aside from mud and dirt, other weathering effects could include the application of a stain or wash to emphasise shadows and bring out the grain or texture of different surfaces such as wood, rock and leather.
Rather than give you a full tutorial on this, I suggest you take a look at some of the excellent videos produced by Dave Youngquist from Last Cavalry on YouTube. There are plenty of really good videos that will explain to you how different weathering effects can be achieved and they are aimed at modellers who have basic weathering skills so they are easy to follow.
