I've always wondered about that. Watercolour are easy to bring out. But I've always known acrylics to require squeezing from tubes of paint, a palette, and a good amount of water to wash brushes quickly after use - It's generally messy. The main problem is that acrylics dry fast so you can't leave them in regular portable watercolour palette/containers which are not air-tight. Once they are dry and hard, you can't paint with them anymore.
So here is my present solution:
Light and portable. Everything fits in the small bag I carry. I can even paint in a nice cafe without messing up things.
I use these small plastic containers from Muji. (http://www.muji.com.sg/store/4547315967667.html?cl=no_color_code_4547315967667) They are air-tight. I leave my acylic paints in them for months and they are remain wet. Because I don't carry a mixing palette around, it helps to pre-mix the colours so it's exactly the colour I like to use most. If I want to mix the colours a little more while I'm out, I simply use the cap of the container. Or a scrap of paper. It also helps to narrow the colours I bring out with me. Reducing the number of colours helps me simplify my pictures. More is less, right?
The colours I carry - White, a pale yellow, a dark brown, yellowish red, light blue, dark blue and black. I mixed in white into my colours so they would look more desaturated. But that's my preference. If you like vibrant colours, leave them strong.
I have a wiping towel, made from an old favourite shirt. And also a spray bottle to dilute the paints with clean water. (The washing water gets dirty after awhile).
As for the brushes, I thought of using a bamboo brush roll so they'd be safe in my bag. But it looked long and cumbersome. So I just used thicker scrap paper, folded it to make it stiff. Then folded it over the brush hairs so they won't bend during transport. And then paper clip it! This fits into my pencil case.
So this is the set up. It might not be suitable for big easel paintings. But it is certainly useful for on-the-go urban sketching trips where you stop at a location, do a quick painting in your sketchbook and move on to the next place.
Here is my list:
1. A5 Sketchbook
2. Brushes + folded paper and clip
3. Small air-tight containers with pre-mixed paint
5. Spray bottle
6. Washing water in a bottle with cap (no spills while on the go) - take note not to accidentally drink.
7. Zip-lock plastic bag to keep the paints and towel in.