Simple gradient issue with OpenGL on iphone simulator - iphone

Question

i was following the tutorial from raywenderlich website, the gradient seems not to work perfectly, is it only because of the iphone simulator, or is it something else? I can't try myself with an iphone. Here is the image :
And the code :
-(CCSprite *)spriteWithColor:(ccColor4F)bgColor textureSize:(float)textureSize {
// 1: Create new CCRenderTexture
CCRenderTexture *rt = [CCRenderTexture renderTextureWithWidth:textureSize height:screenSize.height];
// 2: Call CCRenderTexture:begin
[rt beginWithClear:bgColor.r g:bgColor.g b:bgColor.b a:bgColor.a];
// 3: Draw into the texture
glDisable(GL_TEXTURE_2D);
glDisableClientState(GL_TEXTURE_COORD_ARRAY);
float gradientAlpha = 0.5;
CGPoint vertices[4];
ccColor4F colors[4];
int nVertices = 0;
vertices[nVertices] = CGPointMake(0, 0);
colors[nVertices++] = (ccColor4F){0, 0, 0, 0};
vertices[nVertices] = CGPointMake(textureSize, 0);
colors[nVertices++] = (ccColor4F){0, 0, 0, gradientAlpha};
vertices[nVertices] = CGPointMake(0, screenSize.height);
colors[nVertices++] = (ccColor4F){0, 0, 0, 0};
vertices[nVertices] = CGPointMake(textureSize, screenSize.height);
colors[nVertices++] = (ccColor4F){0, 0, 0, gradientAlpha};
glVertexPointer(2, GL_FLOAT, 0, vertices);
glColorPointer(4, GL_FLOAT, 0, colors);
glDrawArrays(GL_TRIANGLE_STRIP, 0, (GLsizei)nVertices);
glEnableClientState(GL_TEXTURE_COORD_ARRAY);
glEnable(GL_TEXTURE_2D);
// 4: Call CCRenderTexture:end
[rt end];
// 5: Create a new Sprite from the texture
return [CCSprite spriteWithTexture:rt.sprite.texture];
}
Thanks