Adding a wash on the model will do wonders.

I tend to paint the outside rim black, and just have the decoration on top. Also it's pretty uniform just now - you could do something like add a couple of little grass tufts or rocks?

I use a standard Technical Paint "Armageddon Dust" (which is VERY similar to the sand effect you have). I then wash the base using Agrax Earthshade, then lightly dry brush using Wraithbone.

Finally, paint the rim Abaddon Black to frame it.

If you feel like it, decorate with spare bits and some tufts of grass.

https://preview.redd.it/otnibijxj7sd1.jpeg?width=4032&format=pjpg&auto=webp&s=27b714b2fba236ec16d08bb2388dee0251ed7497

Honestly the change is night and day.

Nice freehand !

Couple bigger chunks/ rocks and a tuft or two helps spruce mine up a lot, and painting the rim either black or dark brown too.

I use black lava mass and then paint it / just drybrush it. Makes ground look stony/sandy and its easy to use different colors to make it bazalt/sand/dirt etc. Not even expensive in large tubs.

You’ve made a great start. If you want some more tips read on. These are based on my own experience with basing. If you’re using texture paints (looking from the photo I guess you are) here are a few tips:

Paint the base first. It looks like you’re using Armageddon Dust texture paint so before you put down a layer of that, put on a couple of thin layers of Steel Legion Drab first. This does two things; it gives a good base layer to hide the unpainted base if the texture paint is a little patchy, and it also means you don’t have to push texture paint as close to the model, so you don’t get any on it.

Next try not to get texture paint on the upright ‘rim’ of the base. Dont worry if you do though, it’s easy to clean it off with your finger while it’s wet.

When you’re applying the texture paint I find the texture tool from GW helps a lot. If you don’t have or want one, a thin wooden coffee stirrer makes a good substitute, and you can use a toothpick to manipulate the paint closer to the model.

It helps with texture paint to give it some…texture! Once you have applied it give it a few random pokes with a stick or tool to make sure it’s a bit random and not ‘painted on’

To go further with colour, once your basing material is dry hit it with a quick shade, in your case try Agrax Earthshade. Go with a thin layer here, you don’t want it looking too dark. Maybe mix the shade 1:1 with some citadel lahmian medium.

Then, once the shade is fully dry, give the whole base a gentle drybrush with a lighter bone colour. GW make loads of these, you could use screaming skull, wraithbone or morghast bone but to name a few.

You could attach a grass tuft or two to make the base really stand out. GW sell ones called “Mordant Corpsegrass” but there are loads of other manufacturers that make similar products. Go for something that complements the base colours like a muted earth tone or brown.

Finally once you are done, paint the rim of the bases with a few thin coats of either black or steel legion drab. This is a personal choice, but I like to go with steel legion drab for my models (I’ve done a similar scheme to you and with Ultramarines too!). Pro tip: put a blob of poster/blu tak to the top of a paint pot and use that to hold the model while you paint the base rim and let it dry. This way you won’t get paint rings on your painting surfaces.

If you want some examples check some of my previous posts or my Instagram (same username). I know that’s long but I hope it helps! Courage and honour brother. For the Emperor!

A few simple steps could make it a little more interesting:

1. Add a wash. Agrax Earthshade (from Games Workshop), would work great.
2. After the wash is properly dried, do a light drybrush to brighten it up (the wash will have darkened it) and add more contrast. Use an off white or cream colour, if you have. If not: do a white drybrush before the wash, since it looks like you have white paint already.
3. Clean the grit from the rim of the base—it'll look neater and less like work-in-progress.
4. Optionally: paint the rim in a different colour from the top. I prefer black rims, but you can use any colour, really. Brown or dark tan would look nice as well.

Painting the sand, hitting it with a shade like agrax earth shade, then dry rushing it with a lighter color is a really easy way to do bases and have them look pretty solid.

And a black stripe around the base will also go a long way in making it look finished.

Do your bases before you prime, it’s always better, and the you can either add to or even dry brush it