This video explains how we can specify colors to gallery item based on the type of Item dynamically.
Video explains the concept with Lookup and Color collections on App start.
Please watch full video to learn step by step process.

Sample collection :
ClearCollect(
ItemColorMapping,
{ ItemType: "Laptop", BackgroundColor: Color.Blue },
{ ItemType: "Camera", BackgroundColor: Color.Yellow },
{ ItemType: "Phone", BackgroundColor: Color.Red },
{ ItemType: "Printer", BackgroundColor: Color.Green },
{ ItemType: "Desk", BackgroundColor: Color.Orange },
{ ItemType: "Chair", BackgroundColor: Color.Purple },
{ ItemType: "Scanner", BackgroundColor: Color.Pink },
{ ItemType: "Projector", BackgroundColor: Color.Cyan },
{ ItemType: "Monitor", BackgroundColor: Color.LightBlue },
{ ItemType: "Tablet", BackgroundColor: Color.LightGreen }
)