Very important component of the European roulette strategy is the number of bets. Newcomers are often make a gross mistake, making several bets at the same time. In fact, it is much more profitable to make only one bet in each round.
Let's imagine that you decide to play at odds and bet $1 on every even number and $2 on every odd number. Total in the amount of your costs is $54. If you get an even number, your winnings will be $35 (1*35). If you roll odd, the payout will be $70 (2*35). If it comes up zero, you lose all of that set $54.
If you follow the strategy of one bet, the results of the next round of European roulette will look very different for you. Let's imagine you only place one bet $35 on even numbers only.
I believe the situation is different if you bet with 50/50 probability on both. Let's talk about betting on red and black. Say, you think blacks are going to win and you put 10 dollars on black. At the same time you put 5 dollars on red just in case. You spend 15 dollars. In case black is the winner, you get $20 or $5 of pure profit. I case red wins, you get 10 out of initial 15 dollars back. So, you either win or lose 5 dollars. If you only put 10 dollars on black, you lose them when the outcome is red, but win 10 more if the opposite case. The risk is higher with one bet and I kind of makes some sense to put money on both to lose less. There're some roulette strategies, but I believe that eventually you come to this rare case when something keeps going wrong and you lose everything, just like with martingale.