2 red, 2 black — flat bet, parlay on win.
I just did a small test by betting 2 red, then 2 black (1 unit each). Every time I win, I parlay the next bet (2 units). If parlay win go back to 1 unit
If I lose 4 times in a row: for example, 2 black then 2 red—I wait 1 or 2 spins until the pattern breaks, then start again. I’ll will test it with 10-cent stakes to see if it could be profitable for short-term play.
This apply to all even bets. The strength of this strategy is, that you will win on breaks and long streaks. And waiting after 4 losses till pattern breaks, prevent big losses.