An idea feature for Paint.NET.

[KidBoy140]

...to make text boxes

I don't think there is a text box feature in Paint.NET, although there is an Editable Text Plugin which allows you to change the text after you have first put it on the canvas on a new layer.

...only make one of the letters/numbers/symbols have only one color

There isn't a plugin for this but it is possible. Type your text all in one bright colour, e.g. red. Then use Hue/Saturation to adjust each letter to your liking.

...allow you to change the shape of the text

Don't think this is possible.

...like in Adobe Photoshop

Buy that then.