How change the header color?

(1) In your twentyten theme’s style.css file, find this rule:

#access {
    background: #000;
    display: block;
    float: left;
    margin: 0 auto;
    width: 940px;
}

the black color comes from this property ang change it to your liking: background: #000;

(2) In your twentyten theme’s style.css file, find this rule:

#access a {
    color: #aaa;
    display: block;
    line-height: 38px;
    padding: 0 10px;
    text-decoration: none;
}

font color comes from this property and change it to your liking: color: #aaa;