Why are tasks in Microsoft Project turned red

The short answer... because the MS Project team doesn't have to dog-food their own product.

Really MS Project isn't going to recalculate the rest of the project every time you change the completion of a value (and it may not be appropriate either). It can also screw things up nicely (ask anyone who has accidentally "rebalanced" their project - everything goes fubar)

You can sort of do what you want by re-leveling the resources:

Go to Tools
-> Level Resources
-> (select "Level only within available slack" and click OK)

The best way I've found to do this is to use the build in set of Flag fields, Flag1 up to Flag20. If you have more than 20 resources to color code, this method won't work for you. Note: I'm using Microsoft Project Standard 2013.

We'll start by color coding the 'iOS' resources. Your resource names are assumed to be: iOS, Rails, Design, and QA.

In your Gantt chart view:

1. Add the Flag1 column to your chart. Click the Add New Column button, then type Flag1 and press enter.
2. Right click on the Flag1 column header, and choose Custom Fields.
3. Click the Flag1 field, and click the Rename button. Name it iOS.
4. With the newly renamed iOS field selected, click the Formula button under custom attributes.
5. Type in this formula: IIf([Resource Names]="iOS",Yes,No)
6. Click OK. You should now have a column that says Yes whenever an iOS resource is assigned, and No otherwise.

Now to turn that flag into a color bar.

1. In your Gantt chart view, right click in an open area and choose Bar Styles
2. Click on the Task bar, then the Cut Row button, and the Paste Row button twice. You should now have two bars titled Task.
3. Rename one bar to 'iOS Task'.
4. For the iOS Task, in the Show for ... Tasks column, edit that text to add the text ,Flag1 to the mix. The result for me was Normal,Active,Not Manually Scheduled,Flag1
5. Now click iOS Task, and you can change it's color, text, etc. to suite your style.

Repeat these steps for the other resources.