First of all, what is a ground?
Simply, it’s the first layer that the rest of your painting sits on.
Second, why can’t you just paint on white?
Well, you can! Lots of artists go straight in on a white canvas, or an raw canvas, and paint happily away like that.
However, you’ll find a lot of artists start off with a base layer. Here are a few reasons why:
1) to “lift” the other colours on top. If you are mainly going to paint greens in a landscape, painting a warm colour underneath seems to vibrate with the green and help bring the colours alive. Artists can use burnt umber, fluoro pink, red, orange and yellow. Some even do a split colour - where the top is warm and the bottom is cool.
2) it unifies a painting. Starting off with the same colour all over can help all your colours gel together.
3) it can help you see the lights and darks. If you start with a mid-tone, you’re more likely to get your lights and darks correct. Starting with a white background can mean you don’t see the relationships between the colours so well. However, leaving the paper white is how watercolourists work, so might be worth considering having a white ground if you also work from light to dark.
4) a mid-tone covers the canvas. If you paint thinly, and hate the white of the canvas showing through unexpectedly, a coloured ground can help you make it looks more finished and professional.
5) it gets you started. Having a colour (or many colours) already down helps avoid the blank canvas scenario, akin the writer’s block.
One thing to note though, is that if you want to have a bright yellow or white on the top of a mid-tone, you’ll need to do several coats. One coat to (rule them all), I mean, one coat to lighten the base colour, and then other layers over the top. Yellows tend to be quite transparent, so glazing a yellow over a lighter colour will make it pop, instead of mixing that yellow into white (which makes a pastel yellow).
Having said all that, it’s worth trying it both ways. I’ve been working for a while on a blush pink ground, but feel it might be time to try a lighter one again. We’ll see!