If you're tired of dragging around an eraser in order to clean up any spill over, then set a reference layer! You can do this by clicking the lighthouse icon on the layer that you want to save as a reference. This will then tell the program that further changes to your image will be in relation to that particular layer. I usually set my finished lineart as the reference layer and then flat in my base colors on a separate layer underneath it.
The fill tool in Clip Studio Paint is genuinely amazing. Of course, you have the generic fill setting that this too is set to when opening the program, but did you know there are several other incredible variations in its menu? After setting your line art as your reference layer, make another layer underneath for your base colors. From there, click the fill tool > select Sub Tool > and click Refer Other Layers. Now, you can paint bucket to your heart's content without going outside the lines!
Using the lasso fill tool is a great way to quickly block in shapes and colors! Even though it adheres to the settings that the regular lasso/selection tool does, it is not located under the lasso tool/selection menu! It's actually under the line tool menu near your color preview (the normal icon for this menu is a diagonal line).
I love using this tool for blocking in large shadows. What I recommend doing first is make a new layer, set it to Multiply, change the opacity however you wish, and create a clipping mask by clipping it to the layer below (your base colors). The clipping mask icon looks like a shaded square with an translucent square over it:
You can also do this: right click the layer you want to clip > select Layer Settings (P) > and Clip to Layer Below. That way everything stays tidy and none of your colors from your lassoed shapes will go outside your lines!
If you're stuck trying to figure out a color palette for your image, ask Clip Studio Paint via the Colorize tool! You can ask the program to auto color if you wish, but sometimes you might get a palette that's very off. First, set your Reference Layer (usually your lineart), pick two or three colors, and block them in on another base layer. Then, click Edit > Colorize (Technology preview) > Use hint image and colors > click OK > and process. After a moment, CSP will auto color your image based on the colors that you suggested!
I recommend using this tool as a starting point. It's not the cleanest, so you will need to touch it up a bit and play around with the colors CSP generates, but it is a great tool if you're stuck on what colors you should use or if you want more variation in your hues.
Even though it may feel like cheating, using custom brushes is a huge time-saver. Under the effects tab, you can find trees, clouds, sparkles, petals, and so much more. And these are just the base program brushes! You can add even more custom brushes in the Clip Studio Paint Assets manager. Check out this guide that's all about CSP Assets and downloading them!