Look at a nearly full row in a falling-block game. There is one gap, and the next piece fits. Usually, that is an easy decision.
Now give every block in the piece its own color. The piece still fills the gap, but turning it another way might put three red blocks in a diagonal. Those red blocks could disappear and let a blue block fall into its own match. The same space has become part of two different plans.
That is the idea behind Tetcolor Duo, one of the four games in Tetcolor.
A row does not need matching colors
Duo keeps a familiar rule: fill a horizontal row and it clears. Alongside that, three or more blocks of one color clear when they form a horizontal, vertical, or diagonal line.
Either rule can help when the board gets crowded. You do not have to complete the whole width to remove a small color match, and a mixed-color row is still useful even when none of its colors form a line.
The rules can overlap. If a block belongs to both a full row and a color match, it disappears once and counts once. There is no need to choose which rule to activate.
The next board matters as much as the current one
After blocks disappear, those above the cleared cells move down. Duo checks the board again for full rows and matching colors. If another clear is possible, it happens before the next falling piece.
That is the cascade. The player is preparing a sequence, not just clearing the largest group visible right now.
A modest first match can be worth making if it moves the right block. An impressive clear can also end immediately if the remaining colors do not line up. There is no guarantee that clearing more at once produces a longer chain.
Why keep the simpler games?
Duo is not intended to make the other rules unnecessary. Tetcolor keeps the focus on color matches. Columns fixes the piece shape and lets you cycle three colors. Retris concentrates on complete rows.
Each removes a different decision. That can make a game easier to read, or simply make it the one you feel like playing today. The main menu keeps those choices visible even though its large Play button gives a new visitor a straightforward place to start.
See the difference before playing
The Duo guide includes a short interactive demonstration. A piece falls into a prepared board, the qualifying blocks are highlighted, and the cascade follows. You can pause it and advance through the steps.
For a first attempt, Classic is useful because it leaves out bombs and covers. Once the ordinary cascades make sense, Arcade adds another question: can a bomb reach the blocks you need to remove?
Play Tetcolor in the browser, or start with the rules and demonstrations.