First, change the "Rich Text" setting in the text editor to "HTML" on the post that you want to have the colored text. That is the first thing you should do!

Second, go to this link, type some text, and make a color combination. It's super customizable and easy! Then change the Output Code to "HTML (span tags)" and don't change the Color presets unless it's the fade.

These are the correct settings to make it look like I do it but it's completely color customizable for you.

Third, press the "Generate Color Faded Text" button at the bottom of the page.

Once you get to the page that shows you the results, press the "Select All" button and copy all the text from the "Color Code" box.

Then, come back to Tumblr and switch to the "HTML" tab next to the "Preview" tab and paste the text. Don't paste it in the "Preview" tab or it won't show up.

It should look like this. Don't tamper with it or the fade will look incorrect.

Now, come back to the "Preview" tab and you'll see your colored text like this:

Hello!

From there, you can customize the text like how one would do with normal Tumblr text however you want, like this:

Hello!

And that's the tutorial! I know it's pretty complicated but it gets easier the more you practice. You can experiment as much as you'd like too! :3