When using breadboards / stripboard / Fritzing / etc, are there conventions for which colour wires to use?
The following make sense to me:
Gnd = Black
V(supply) = Red
Q1, Q2, Q3, Q4, etc = Brown, Red, Orange, Yellow, etc
But are there any other conventions or is it all arbitrary?
And what is good practice if you have 12v, 9v and 5v all on the same circuit?
I know of no convention, but I know that using a consistent system will help a lot toward avoiding wiring errors. Like you I use black for ground, red for 5V. I use yellow for 12V and orange for 3.3V. The usually colors for pin numbers from the resistor color code. So a wire from pin 3 of the Arduino would be orange, pin 4 is yellow, etc.
My point is to develop a system and keep to it when possible.
I also this. It's also very typical in most systems I see.
This!
As you see above there is no "official" color code. The only absolutely common code I know of is black being ground.
I use some devices that have regulators so I deal with 12v. MY PERSONAL color code is:
Red = 12V
Orange = 5V
Yellow = 3.3v
Black = common (ground)
Gray = SCL (for I2C)
Blue = SDA (for I2C)
Remember the color code you choose is to help you build (without mistakes) and troubleshoot your hardware.
I will admit if my hardware does not use 12V and I need colors for other signals I will change to
Red = 5V
Orange = 3.3V
Yellow is some signal (usually reset if reset is needed)
Remainder the same.
If you are using I2C regularly I found the Gray/blue to be extremely helpful when wiring hardware. I no longer even have to think about which is SCL and which is SDA. Oh you could use any two colors as long an you are consistent and write it on a post-it at your bench until its ingrained in you brain 
I don't have too many jumper wires so I generally use multiple colors for the same thing.
Red/Yellow/Green = Any power (3.3V, 5V, 12V, etc)
Black/Grey/Brown = Ground
Thanks all.
It sounds like I've inadvertently adopted the above standards.
I use color of match the ATX power supply that is the power supply of the PC.
GND : Black
+5V : Red
+12V : Yellow
+3.3V : Orange
I use above for the power line.
Anything can work, as long as you are consistent. Just don't do it backwards, or contrary to widely used conventions, like red for ground and black for vcc, unless you want to find yourself in trouble.
Years ago I built a kit which used 5 different supply voltages, some positive and some negative. I used rainbow ribbon cable to distribute this and used the progression red-orange-yellow-green-blue for the voltages from most positive to most negative. Plus black for ground.
I tend to still use warm colours for +ve supplies, cold colours for -ve supplies, which is compatible with red/black for the case of only +ve supply.
I can't remember what old ATX supplies used for the +12V/+5V/+3.3V/-5V/-12V wires, but I suspect it was something similar.
The only wiring colour code I know of it the "BT standard colour code" used for telephones in the UK.
A limited sub set is used when a telephone socket is connected in your house, but the full colour set uses striped wire and that allowed hundreds of numbers to be defined based on the stripe and background colour.
That's ingenious.
+1 for the telephone wire. I have been using it for breadboarding for many years.
It's not just the telecoms industry. Regular readers will know telecoms is my industry, but my brother worked in power stations. My brother told me that power stations use the same cable and the same colour code for control signals.
My next door neighbor worked for the power company. He he gave me some control cable. I don't recall the colors but the cable was 5 X #12 !
Several years ago there was a telephone lineman working on our street. I stopped and asked him if he could spare a few feet of the, probably, 50 pair 24 gauge stuff that they use. He said sure and gave my a nearly empty spool with, must have been, 20 feet on it. I doubt that I will run out of it in my lifetime.
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Since I found the multistrand silicone wire one could by on ebay, I've used nothing else. This stuff has many strands, I think the #22 has 50 strands. The results is the wire is limp. It does not move the boards or sensors I've used it on and it doesn't melt when soldering it to a board.
I use #28 for signals and #24 for power.
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One "oddity"...
With AC power wiring in your house, here in the U.S., black is "hot" (120VAC).
Green is earth ground (no current flows unless there is a fault).
White is Neutral (AC current "return", grounded at the circuit breaker box).
Red is 120VAC (relative to Neutral) opposite phase of the black for 240V circuits.
(I don't know about 3-phase wiring but that's not normal in a house.)
A particularly odd convention. Making the least obvious colour, the most dangerous.
The Euro standard - brown for live, blue for neutral - also appears somewhat nonintuitive, but was apparently designed to suit protanopes.
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