It depends on your software.
99% are just showing:
if the last price on the tape is less than the previous last price (n < n-1) then it will be displayed in red.
If the last price on the tape is greater than the previous last price (n > n-1) then it will be displayed in green.
You are seeing red for a downtick, green for an uptick. Generally an uptick is because someone lifted prices to hit an offer (ask), or prices fell because someone downticked prices to hit a bid.
I say generally, because that's not always the case. Especially in markets where there is variable spread (e.g.
DAX, most thinner futures/stocks), it can change. Let me try and draw an example.
A = Offer/Ask
B = Bid
*** = Traded Price
1.
A
A
A
A
B*** <--- last traded price at the bid.
B
B
B
2.
A
A
A
A*** <--- last traded price at the ask. Price is "upticked" by TWO ticks.
<--- SPREAD (the gap) also widens as traders pull their offers up.
B <--- Previous last traded price at (1)
B
B
B
3.
A
A
A <--- previous traded price level at (2)
A*** <--- last traded price at the ask. Price is "downticked". Someone inserts an Ask into the Spread, and it's hit.
B (spread has closed).
B
B
B
In part 3, price is downticked as well as traded at the ask / offer.
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If you are trying to learn to read tape, I highly recommend reading straight from a good price ladder where you can see the bids and offers as well.
That way, you see the whole picture:
Implied Supply
Supply
Equilibrium
Demand
Implied Demand
Cheers.