On your order layout, use the portal tool to draw a box where you want to place it. Select the LineItems table in the "show related records from" drop down. In most cases, you would also double click the line between Orders and LineItems and specify "Allow Creation of Records via this relationship" for lineitems so that you can add items to the order simply be selecting a product ID in the portal.
You might compare your set up to this simple demo file created by Comment: http://fmforums.com/forum/showpost.php?post/309136/
That would be expected, but won't you want to add a field in LineItems for selecting the color for a given item?
This, BTW, is a key design issue for you to look at know. There are two approaches you can use here, one ProductID with many possible colors or you can give each color option for a given product it's own unique ID. With a unique ID for each color, selecting the item ID specifies the color. This is especially useful when setting up a point of sale system with a barcode scanner to scan your products if the barcode uses these color specific product IDs.
If you don't want to use color specific product ID's, you can set up a conditional value list where you get a drop down list of only those colors specified for the selected Product.
Make copies of the table occurrences for Colors and ProductColors and link them in to RelatedProducts. Let's say the new TO of Colors is called RelatedColors. Define a value list with the specify field option and list colors from RelatedColors. THen specify the "Include only related values starting from" option and select LineItems from the drop down. Now you should have a value list of only the colors possible for a given item, provided you select the ProductID before selecting the color.
To learn more about conditional value lists:
Forum Tutorial: Custom Value List?
Knowledgebase article: http://help.filemaker.com/app/answers/detail/a_id/5833/kw/conditional%20value%20list