Simple function FadeBetween to interpolate between two colours. Small demo in existing example.

examples/shapes/shapes_rectangle_scaling.c

@@ -12,6 +12,7 @@
 ********************************************************************************************/
 
 #include "raylib.h"
+#include <math.h>
 
 #define MOUSE_SCALE_MARK_SIZE   12
 
@@ -31,6 +32,11 @@ int main(void)
     bool mouseScaleReady = false;
     bool mouseScaleMode = false;
 
+    Color from = LIME;
+    Color to = GREEN;
+    Color color = from;
+    float t = 0.0f;
+
     SetTargetFPS(60);               // Set our game to run at 60 frames-per-second
     //--------------------------------------------------------------------------------------
 
@@ -39,6 +45,8 @@ int main(void)
     {
         // Update
         //----------------------------------------------------------------------------------
+        double time = GetTime();
+
         mousePosition = GetMousePosition();
 
         if (CheckCollisionPointRec(mousePosition, rec) &&
@@ -59,6 +67,9 @@ int main(void)
             if (rec.width < MOUSE_SCALE_MARK_SIZE) rec.width = MOUSE_SCALE_MARK_SIZE;
             if (rec.height < MOUSE_SCALE_MARK_SIZE) rec.height = MOUSE_SCALE_MARK_SIZE;
 
+            t = 0.5f * sin(time * 4.0f) + 0.5f;
+            color = FadeBetween(from, to, t);
+
             if (IsMouseButtonReleased(MOUSE_LEFT_BUTTON)) mouseScaleMode = false;
         }
         //----------------------------------------------------------------------------------
@@ -71,7 +82,7 @@ int main(void)
 
             DrawText("Scale rectangle dragging from bottom-right corner!", 10, 10, 20, GRAY);
 
-            DrawRectangleRec(rec, Fade(GREEN, 0.5f));
+            DrawRectangleRec(rec, color);
 
             if (mouseScaleReady)
             {

src/raylib.h

@@ -1276,6 +1276,7 @@ RLAPI void DrawTexturePoly(Texture2D texture, Vector2 center, Vector2 *points, V
 
 // Color/pixel related functions
 RLAPI Color Fade(Color color, float alpha);                                 // Returns color with alpha applied, alpha goes from 0.0f to 1.0f
+RLAPI Color FadeBetween(Color from, Color to, float t);                     // Returns color interpolated between two inputs by t 0.0f to 1.0f
 RLAPI int ColorToInt(Color color);                                          // Returns hexadecimal value for a Color
 RLAPI Vector4 ColorNormalize(Color color);                                  // Returns Color normalized as float [0..1]
 RLAPI Color ColorFromNormalized(Vector4 normalized);                        // Returns Color from normalized values [0..1]

src/textures.c

@@ -3552,6 +3552,22 @@ Color Fade(Color color, float alpha)
     return (Color){color.r, color.g, color.b, (unsigned char)(255.0f*alpha)};
 }
 
+// Returns color interpolated between two inputs by t 0.0f to 1.0f
+Color FadeBetween(Color from, Color to, float t)
+{
+    if (t < 0.0f) t = 0.0f;
+    if (t > 1.0f) t = 1.0f;
+
+    Color out;
+
+    out.r = (unsigned char) (((1.0f - t) * from.r) + (t * to.r));
+    out.g = (unsigned char) (((1.0f - t) * from.g) + (t * to.g));
+    out.b = (unsigned char) (((1.0f - t) * from.b) + (t * to.b));
+    out.a = (unsigned char) (((1.0f - t) * from.a) + (t * to.a));
+
+    return out;
+}
+
 // Returns hexadecimal value for a Color
 int ColorToInt(Color color)
 {