Let me start by saying my advice in this post comes from being a Mac user. If you're on a PC, the exact way you go about doing it might be a little different.
I personally use Graphic Converter for this kind of work, but almost any image-manipulation program will do. First, you need to use Mission Computer to export the sprite's base image into some sort of image format such as .png, .PICT, etc.* Then load it into your image-manipulation program. Find the program's Hue Edit tool, whatever it may be called. In the case of Graphic Converter specifically, this option is located in the menu bar. Go to Filter > Color Adjustment > Hue Adjust. Then just fiddle with the slider till you find something at least close to the color you want. If you're not quite happy with the end effect, you can use another tool Graphic Converter has to fine-tune the hue effect. Go to Picture > Brightness/Contrast/Hue. The hue option here is a bit limited compared to the first one I mentioned, so it's best used for fine-tuning the final color. You can also use the brightness, contrast, and saturation sliders to fiddle with the image—make it brighter, darker, etc—till you get something you like.
Now you need to put the image back into EVN. First you need to make sure the background is completely black. Do this by double-clicking on the paint bucket tool in the quick-select that should be to the right of the image. Change its 'Tolerance' to 1 and make sure your selected color is complete black. Click on the background and viola, black background. Save the image. This is important as you're not done editing it yet.
Now you need to make the sprite mask. Double-click the Magic Pen tool ('Magic Wand' is trademarked by Photoshop) and change its tolerance to 1, as well. Click on the background. This should select the entire background but no part of the sprite itself. Go to the tool bar at the top and click Edit > Invert Selection. This will switch your selection to the sprite instead of the background. Then simply hit the delete/backspace key to get rid of the sprite, leaving a white space in its place. Save this as a separate image. Now, assuming you know how to import sprites into EVN, you should be set.
Hope this helps! I'm afraid I've never been too good at giving tutorials, so if you need me to clarify anything, let me know and I'll try.
- I'm not sure if the Windows beta of Mission Computer can export sprites yet. If not, you'll have to find another way. Unfortunately, I've no idea what to use on Windows.