I was wondering if it is possible to have two words in the header (main text on WordPress blog) be two colors?
I think you can do this with CSS, try this:
#site-title a:after {
content: " Hello!";
color: green;"
}
Also, you could make the site title one color and the tagline another color (just an alternative option).
I do have a psd-file.
What steps do I need to take to have this image CSS'ed in the header?
You must convert your PSD file to a PNG, GIF, or JPG file before uploading. You can use Appearance → Header to add it to your site without using CSS if the preset size of header image that comes with the theme works for you. In Twenty Ten, the header image size is 940 × 198 pixels.
... and how does an image and header affect being found and indexed into search engines, whilst this is not text (and searchable?) anymore...
If you shift the text left instead of hiding it using display:none, it shouldn't make any difference for search engines.