What's wrong with this code which checks for 2 colourable graph (bipartite)

I wrote a code to check whether a given graph is bipartite or not, but not sure why one code works but not the other.

The below doesn't work,

bool dfs(int i, int color, vector<vector<int>> &graph) {
    if (visited[i] == -1) {
        visited[i] = color;
        for (int adj: graph[i])
            return dfs(adj, !color, graph);
    }
    return (visited[i] == color);
}
bool isBipartite(vector<vector<int>> &graph) {
    visited.assign(105, -1);
    for (int i = 0; i < graph.size(); i++) {
        if (visited[i] == -1)
            if (!dfs(i, 0, graph)) return false;
    }
    return true;
}
vector<int> visited;

But when I change above to this

bool dfs(int i, int color, vector<vector<int>> &graph) {
    if (visited[i] == -1) {
        visited[i] = color;
        for (int adj: graph[i])
            if (!dfs(adj, !color, graph)) {
                return false;
            }
    }
    return (visited[i] == color);

}
bool isBipartite(vector<vector<int>> &graph) {
    visited.assign(105, -1);
    for (int i = 0; i < graph.size(); i++) {
        if (visited[i] == -1)
            if (!dfs(i, 0, graph)) return false;
    }
    return true;
}
vector<int> visited;

it Works, Can anyone tell why ? Notice that only line 5th and 6th in dfs function got changed.

shouldn't both code work since they seem to work same ?

In your first code snippet,

        for (int adj: graph[i])
            return dfs(adj, !color, graph);

what happens is you're returning within the first iteration of the for-loop. So for each node, you're only checking one of its neighbors, and ignoring the rest.

Note that your second snippet is not entirely correct either. You're missing a case where the node has no neighbors, or all of its neighbors have correct colors:

bool dfs(int i, int color, vector<vector<int>> &graph) {
    if (visited[i] == -1) {
        visited[i] = color;
        for (int adj: graph[i])
            if (!dfs(adj, !color, graph)) {
                return false;
            }
        return true;
    }
    return (visited[i] == color);
}