For colored boxes that stand out from the tab or sheet background insert a text object (Layout->New Sheet Object->Text Object). In the object properties choose a background color that works well with the sheet background. Drag the text object around and adjust the size with your mouse. Whether you actually have text in your text object is up to you. The text objects can be overlaid or layered with other sheet objects like charts. This is a common way to add a legend or explanation of the data that will appear alongside a chart. The Layout tab in the object properties has a Layers setting that will help with overlapping objects. The transparency setting can also help with layered objects. Don't hesitate to click the Help button for more information if you see something that you don't understand.
The background of the tab or sheet can be changed by right-clicking on the top of the sheet where the tab label is. Then select Sheet Properties and look for the controls that adjust background color or let you add an image. A background photograph is often too busy for a sheet containing chart objects but it may be perfect for a title sheet. The background and colors of a well-designed website might give you some ideas for how to design the sheets of your document.
The text object can show a diagram or picture. This is an easy way to add a corporate or department or project logo to the document. When you add the text object look at the General tab of the object properties. In the Background area click the Image button and then the Change button (btw- note how QlikView uses lines and boxes to group the logical elements of the properties window… a few lines can make the information on your document easier to understand too). Then you can select a jpg or png file from your computer and insert it into the document. Try it out! Even if you are designing a business document there's no need to be boring. Insert a cartoon image of your boss... but before the document is seen by company management you should make sure the image is respectable and that you or your company own the rights. Consider this method for adding a diagram to your document or adding photos of products or a map of locations.
Text objects can be set up so that they appear or are invisible based on the data. On the text object Layout tab you can click the Conditional button and then enter an expression that will control whether the text box is visible or not. You could make the text object visible based on the person using the report, the loaded data, time of day, or day of the week. Use it to alternate between two or more different text objects with different images. For example, in a corporate dashboard application you could make the image of a sunny day or of a stormy day appear based on the current month's revenue versus last month (or whatever passes for good news versus bad news in your business). The expression should be something that evaluates to a zero or a non-zero value. If the value is zero then the text object is not visible.